Before I start to talk about GUI API, I want to tell you that GUI API was not developed by us. We’re only the web the writing about mods, texture packs, skins, and any other things about Minecraft.

Then… who developed GUI API ? His name is Lahwran (Minecraftforum’s username) I want to thanks him for this great API, if there is no you then, there will be lesser mod in Minecraft’s community.
Now you knew everything you have to know, let’s get back and talk about GUI API. GUI API or Graphical User Interface API was developed to be use for developers or modders, to create a better user interface in Minecraft.