I've got to admit that I don't have the slightes clue how the xbox360 gui looks like. What would be beneficial about having a clone on the gp2x? Is it even useful?
The builtin menu can be replaced quite easily, but if you screw it up, your gp2x is toast until you run a firmware update or repair via a serial connection.
Easier way would be to use the "autorun" functionality of the existing menu.