Vanilla GUIs posted by Runakai (17869 views).
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Vanilla GUIs ~ Create your own little world in a prison called inventory

Are you looking for a good way of making GUIs in Skript? Then you're more than welcome to read this tutorial. If you have used vanilla GUIs before I recommend to read this page again since it will include some useful information and will only require vanilla Skript now since we're approaching them differently now. No need for Skript-mirror, SkQuery or Sk-Bee to get the inventory name. We don't need them. Not even on 1.8! The only thing you need on 1.8 is to see a doctor.
If Metadata comparison does not work for you and you're on 1.12, you need to use a snippet that's provided at the bottom of the tutorial!

Preperation

Okay, let me explain how vanilla GUIs work by describing it. You open an inventory and set the slots. Every time you click on the slot you check if certain conditions are met. Based on that you perform effects. You didn't understand that? Look at that:

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

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 shor-term based of a single entity or block.
In our case we use it to distinguish inventories.

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:


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

Let's make a bit of code!

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)

addSpoiler: Get Hotkey Clicking

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

addSpoiler: 1.12 metadata fix

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

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.  
11. [/COLOR]
12.  

Thanks for mr.gigi for providing a workaround!