I have one of those "Arcade" adapter boards, and it works okay with my IIgs. (I'm not going to say "great", just "okay". It looks fine with games but the GS OS desktop doesn't look that great. Better than it does with a composite monitor, however.) Remember of course you'd be wiring that up to the IIgs' RGB output.

Re: converting one of those 12" Macintosh RGBs, as people have noted they run at a different scan rate and they're not multisync at all. Since they *are* relatively simple fixed-frequency monitors I imagine it would be *possible* to convert one to run at the IIgs' NTSC Hsync, but it would involve changing components and would be something well beyond my expertise to comment on.

(A skilled TV repairman with access to the schematics could probably tell you how feasible the idea would be.)