GEM screen resolution depends on the capabilities of the graphics card, at that time (1985) 640x200 was quite popular. I see there https://www.seasip.info/Gem/Drivers/video.html drivers for GEM 1.x for e.g. Hercules 720x348
Actually PC GEM 1.1 in 640x200 wasn't so bad:
Digital Research Inc designed GEM in that way that, based on XEROX experiences.
Regarding green desktop, if I remember correctly the first 16 colors in GEM were standardized by Digital Research. Blue was reserved for PC GEM, therefore Atari didn't have much choice:
PC GEM:
ST GEM: