The BeOS version requires X. The native BeOS port was never finsihed, AFAIK.
I think that on AmigaOS we should have a rootless X-Window server (with a window manager more or less integrated with AmigaOS) before GTK because I guess that it may be easier to port GTK and other stuff later and the maintenance of the GTK port (that would require X-Windows) would be easier.
Take it as a constructive criticism and just as an idea to reduce the amount of work required to port GTK... Porting GTK is a brave effort, but a rootless XWindow server like the MacOS X one would help us a lot and may be easier.
Good luck with the project, I hope that it succeeds, but please think about the X-Window server idea... (maybe for the next project? ;-)