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 1.6.2
http://dl.mod-minecraft.net/download.php?file=001393ce078f01b95f254f4799d46fd7
For 1.5.2
http://dl.mod-minecraft.net/download.php?file=006c40f3e72c2ad7eac4021e85c3351b
For 1.5.1
http://dl.mod-minecraft.net/download.php?file=184cda341674c61c4cad3bf964da66e7
For 1.4.7/1.4.6
http://dl.mod-minecraft.net/download.php?file=110b0aa025c95707ddaa02b89d4f72f5
For 1.4.5
http://dl.mod-minecraft.net/download.php?file=2003c710a1dfe89c9804e00d21889468
Credits: ShaRose