For Users:
GuiAPI is a coremod, and thus requires Minecraft Forge.
Installation:
- Download the GUI API
- Drag all the contents from the extracted file into the minecraft.jar
- Delete META INF
For Minecraft 1.6.2
https://minecraftlucky.com/dowload/file-1612
For Minecraft 1.5.2
https://minecraftlucky.com/dowload/file-1615
For Minecraft 1.5.1
https://minecraftlucky.com/dowload/file-1618
For 1.4.7/1.4.6
For Minecraft 1.4.5