>>105964758
GEOS was just a windowing layer on top of the OS
there was/is one for DOS as well, basically a competitor to Win3.0 (although I'm unsure of the time frame - probably too little too late). limited to 16-bit real mode apps AFAIK

the original DOS AOL client shipped with it. probably one of the few widely-used 3rd party apps to use it - only one I ever personally saw, anyway