But a game GUI doesn't need to interact with Windows (thank god).
Believe it or not, some people actually write level editors and such for their games, and some people like to make their game moddable by their EUs in which case, you would need editors for that. I am not sure why a select few choose to cower under the bed and wet their pants in fear over a GUI module being released for those who wish to use BMax as a professional development tool.