You are looking for a good way of making GUIs in Skript? Without having to update an Addon or possibly risk that it's going to get unmaintained? Well, you cant stop searching now!​

Because vanilla GUIs are integrated into Skript. That's how the name origins from. It's a vanilla feature from Skript. I will introduce you to the basic mechanichs of vanilla GUIs and show you multiple ways on how to set them up.
(PS: Best practice would be to go with metadata-values. You'll get what that is in a few minutes.)

In order to use vanilla GUIs it's always good to use the latest Skript version for your respective Minecraft version.

Spoiler: Versions

If you're still using 1.8.8 (Why? Update. Seriously. Gross.) you should be using dev36.
Link: https://github.com/SkriptLang/Skript/releases/download/dev36/Skript.jar
Link: https://github.com/Matocolotoe/Skript-1.8/releases/tag/2.5.3
(You may be wondering why there are 2 link! Gios fork (the bottom one) is recommended since it provides more features. Both work though.)

If you're using anything above 1.8.8 but below 1.13, so up to 1.12 you should be using Skript 2.6:
Link: https://github.com/SkriptLang/Skript/releases/tag/2.6
(this may be changed as of now this is the latest working version. It's going to be discontinued working for 1.12 on the next update.)

For anything above 1.12 the latest Skript version is recommended.
Link: https://github.com/SkriptLang/Skript/releases

Let me first describe to you in words how vanilla guis work.
Open an inventory
Set the slots however you like
Check every time a user clicks within the inventory, and do something based on that.​

There are 3 ways to set up vanilla GUIs that basically work the same.

Spoiler: First way: Metadata

The first method that should be prioritized is using metadata.

Spoiler: What is Metadata?

Metadata means data within data. So basically it is like using variables. The only difference is that metadata have a holder that's usually an entity or a block and it gets deleted when the server stops since it's saved in RAM. Skript's metadata implementation can only store single values per metadata key per holder. So no lists are possible. It is great for saving information short-term based of a single entity or block.
In our case we use it to distinguish inventories.

I will just show you an example of opening inventories with metadata.

Code (Text):

1. command /vanillaGUI:
2.     description: The best way of doing something.
3.     trigger:
4.         set metadata tag "vanillaGUI" of player to chest inventory with 3 rows named "&7Hello There"
5.         set slot 0 of metadata tag "vanillaGUI" of player to stone named "&6General Kenobi"
6.         open (metadata tag "vanillaGUI" of player) to player

Easy, right? Words confuse. Code doesn't.
One thing you will encounter while running that code is that you're able to steal your precious stone. You don't want that, right! Can't let players have fun huh.

Now we're doing the "check if player is clicking" thing. Trust me, it's easy.

Code (Text):

1. on inventory click:
2.     #check if the clicked inventory is the one we just created
3.     if event-inventory = (metadata tag "vanillaGUI" of player):
4.         #cancel the clicking. This makes the Item unstealable
5.         cancel event

Et voilà you're done!

Now if you include both pieces of code in your script file you're good to go. Your stone is unstealable and your players are frustrated.

You can learn a bit more about the inventory click event,

Spoiler: Inventory Click Event

To perform different actions you use the Inventory click event, add some conditions and based on that build your own little actions for individual slots OR the entire inventory!
Let's say you want to cancel every click in an inventory without checking each individual slot every time. Simply use this:

Code (Text):

1. on inventory click:
2.     if event-inventory = (metadata tag "vanillaGUI" of player):
3.         #check if the clicked inventory is the one we just created
4.         cancel event
5.         #cancel the clicking. This makes the Item unstealable

Pretty self-explanatory, right? Cancel the event, cancel stealing.
The event-values for this event are:

Spoiler: Event-Values

Event-item:
Get the TYPE of an Item. If you click on a stone named "General Kenobi" this will only return stone.
Event-slot:
Get the clicked item including name etc.
Note: if you debug this expression solely, it will return something along "inventory slot X of Y". You can use it in a condition just fine
If event-slot is diamond named "X":
But if you want to get it in a send effect or similar, set a variable to the event-slot and use that variable.
Index of event-slot:
The index expression is universal but we use it to get the clicked slot. Numbers rangin from 0-54
Event-click type:
This is by far the most interesting one. Get the way a player clicked on an Item! All of them can be found here:
https://docs.skunity.com/syntax/search/id:5297
Event-inventory action:
Basically the same as event-click type, just with more options and possibilities such as detecting if a player picked up all of the items or half of the stack etc. List can be found here:
https://docs.skunity.com/syntax/search/id:5153
Event-Inventory:
Get the type of the event-inventory. Ranging from Hoppers to Chests up to Brewing stands etc. A list of them can be found here:
https://docs.skunity.com/syntax/search/id:5321

Remember how I said there are 3 different ways to make vanilla GUIs? I will not describe them much since as I said, they are very similar.
You still open an inventory and see within the inventory click event if it's the desired one.

Spoiler: Second way: Using variables

Instead of using metadata we're using variables. Same thing basically.

Code (Skript):

1. command /vanillaGUI:
2.     description: The second best way of doing something.
3.     trigger:
4.         set {Inventory::%player%} to chest inventory with 1 row named "Vanilla!"
5.         set slot 0 of {Inventory::%player%} to stone named "&6General Kenobi"
6.         open {Inventory::%player%} to player
7.  
8. on inventory click:
9.     #check if the clicked inventory is the one we just created
10.     if event-inventory is {Inventory::%player%}:
11.         #cancel the clicking. This makes the Item unstealable
12.         cancel event

Nothing more to explain. You set a variable now instead of metadata.

Spoiler: Third way: Just using inventory names

You know if you're lasy, this is perfectly fine to use. Instead of setting a holder (variable or metadata) to an inventory, you just open it.

Code (Skript):

1. command /vanillaGUI:
2.     description: The laziest best way of doing something.
3.     trigger:
4.        open chest inventory with 4 rows named "&cLazy gurl." to player
5.        set slot 0 of player's current inventory to stone named "&6General Kenobi"
6.  
7. on inventory click:
8.     #check if the clicked inventory is the one we just created
9.     if name of event-inventory is "&cLazy gurl.":
10.         #cancel the clicking. This makes the Item unstealable
11.         cancel event

Note that this method does not work on 1.14 unless you are using SkBee. You shouldn't be on 1.14 anyway since server performance is baaad.

The following code is just an example on how to use this system.

Code (Text):

1. command /vanillaGUI:
2.     description: The best way of doing something.
3.     trigger:
4.         set metadata tag "vanillaGUI" of player to chest inventory with 3 rows# named "&7Hello There"
5.         set slot 0 of metadata tag "vanillaGUI" of player to stone named "&6General Kenobi"
6.         set slot 1 of metadata tag "vanillaGUI" of player to player's skull named "&6%player%" with lore "&7Right click me to receive my head!"
7.         open (metadata tag "vanillaGUI" of player) to player
8.  
9. on inventory click:
10.     if event-inventory = (metadata tag "vanillaGUI" of player):
11.         #check if the clicked inventory is the one we just created
12.         cancel event
13.         #cancel the clicking. This makes the Item unstealable
14.         if index of event-slot is 0:
15.             #check if the clicked slot is the stone, so 0
16.             give event-slot to player
17.             #give the clicked item to the player.
18.         else if index of event-slot is 1:
19.             #check if the clicked slot is the skull, so 1
20.             if event-click type is right mouse button:
21.                 #check if the player right clicks the item
22.                 set {_playerHead} to uncolored name of event-slot parsed as player
23.                 give event-slot to player
24.                 send "&6Player %player% has received a skull of you!" to {_playerHead}
25.                 #since we can't use arguments, we just parse the name of the event-slot and get the skull by doing that.
26.                 #you can use the variable as you like. I simply sent a plain text to the player.
27.                 #ban {_playerHead} would also work
28.                 #using NBT will work as well!

If you have any questions let me know in the comment section.

Some Snippets. Some of them require skript-reflect Since it's the most superior add-on you should have it installed anyway. The metadata fix doesn't require skript-reflect.
(Skript-reflect is a fork of skript-mirror)

Spoiler: Get Hotkey Clicking

Code (Text):

1. import:
2.     org.bukkit.event.inventory.InventoryClickEvent
3. expression [event( |-)]([number ]key|hot[[ ]bar][ key])[ button]:
4.     return type: integer
5.     get:
6.         return event.getHotbarButton() + 1 if event.getHotbarButton() != -1
7.         return -1
8.  
9. expression [event( |-)]([number ]key|hot[[ ]bar][ key])[ button]( |-)slot:
10.     return type: integer
11.     get:
12.         return event.getHotbarButton() if event.getHotbarButton() != -1
13.         return -1

Spoiler: 1.12 metadata fix

Code (Text):

1. function compare(1: inventory, 2: inventory) :: boolean:
2.     return false if (name of {_1}) != (name of {_2})
3.     return false if (rows of {_1}) != (rows of {_2})
4.     return false if type of {_1} != type of {_2}
5.     return true

Code (Text):

1.  
2. command /gui:
3.     trigger:
4.         set metadata tag "vanillaGUI" of player to chest inventory with 3 rows named "Best GUIs"
5.         set slot 1 of (metadata tag "vanillaGUI" of player) to diamond hoe named "&d:("
6.         open (metadata tag "vanillaGUI" of player) to player
7. on inventory click:
8.     if compare(metadata tag "vanillaGUI" of player, event-inventory) = true:
9.         cancel event
10.  

Thanks to mr.gigi for providing a workaround!

 

That's something that appeared to me as well, should've mentioned it in the tutorial, you're right! Normally every click should get cancelled however, sometimes it doesn't. I fixed that by adding a condition to see what kind of click type was still able to steal the item and cancel that. That's annoying, I know but it doesn't appear for everyone! On my localhost server it seemed to work perfectly fine on my hosted server however it didn't. Maybe installing newer versions if skript/paper fixes the issue

 

May show me the code?

 

I don't know then, the "ugly" way of cancelling would be to set a variable to the cursor slot, set the cursor slot to air and then setting the original slot to the item in the variable

 

You do not have a target entity in the inventory click event. Your uuid local variable is not set in the inventory click

 

Hey, could you provide us with code?

 

Could you provide us with source?

 

I still see people having problems on 1.12 and metadata. I might want to rephrase that warning but I'll not remove it.

--- Double Post Merged, Nov 15, 2020, Original Post Date: Nov 15, 2020 ---

It's the other way around. Vanilla skript will always be faster than using addons

 

Hey, it's save to use vanilla GUIs. It's the main method of doing GUIs anywhere and they're save