Ah, I see. That explains why it sounds obscure.
My suggestion would be to start trying some converters:
http://www.3dlinks.com/3D_Converters.cfm
Most are "free" (though to get a full version with more formats, you do have to enter a registration code.) But I'd suggest sorting them by rating and checking out some of the top rated ones. Most of the "free" ones will at least convert to .obj, but I'm not sure about GBX. But they should have supported formats listed on the sites.