In order to make GUIs you can copy and paste my pack named _iainternal and edit it on your likings:contents/_iainternal/resourcepack/assets/_iainternal/textures/gui/
In order to create custom GUIs you can use the JavaAPI or install a plugin like ChestCommands.
If you are using a plugin like ChestCommands you have to name the GUI like this:
:offset_-16::blank_menu:
This tells ItemsAdder to replace the GUI name with a -16 offset special character that will shift the blank_menu texture of 16 pixels back.
Example addon
Here you can get an addon example
https://www.spigotmc.org/resources/gui-server-rules-main-menu-itemsadder-addon.84793/
Example content created by users (Copyright by Gust)
