Apple monitors

Correct. Macintosh monitors are TOTALLY different from the Apple IIgs RGB monitor. The IIgs needs a 15.75khz RGB monitor - essentially NTSC rate - and isn't compatible with later monitors. Your best bet is the 12" RGB monitor that goes with the IIgs. Or, if you don't have one, an early NEC multisync (original, 2a, etc) will work, as will a Commodore Amiga monitor, or even an Atari ST monitor. Note that all non-Apple monitors will need a homemade cable to connect to the IIgs. Not only are the frequencies different on the IIgs and the Mac monitors, but the pinouts are different as well. You can also use a color or monochrome composite monitor, but the image will be a bit fuzzy - RGB is really your best bet. If you really need something bigger than the stock RGB monitor (say, for a presentation), an arcade machine monitor will also work - and those are available as 13", 19" and 25" fairly regularly. Note that you'll need to power an arcade monitor from an isolation transformer. Also note that a bigger monitor won't get you any advantage other than a huge picture - you can't do any better screen resolution than what it's doing right now.