While the IIgs video and Mac video both decent from the Apple III video (connector) and even look superficial the same, the most important difference is how sync signals are handled. Where the Mac handles each sync signal seperate, the IIgs interface imposed sync on the colour signals.
Pin IIgs signal Mac Signal
1 Signal ground (Red) Signal ground (Red)
2 Analog RED with sync Analog RED
3 Composite sync CSYNC
4 No connection Monitor sense 0
5 Analog GREEN with sync Analog GREEN
6 Signal ground (Green) Signal ground (Green)
7 -5 volts DC Monitor sense 1
8 +12 volts DC No connection
9 Analog BLUE with sync Analog BLUE
10 No connection Monitor sense 2
11 Sound Signal ground (CSYNC/VSYNC)
12 NTSC/composite color out VSYNC
13 Signal ground (Blue) Signal ground (Blue)
14 No connection HSYNC ground
15 No connection HSYNC
Are there any 8 or 16-bit systems that could work with this monitor? Adapters and cables are fine if necessary even if they degrade the signal.
Then many are possible in theory. In reality it's not just signal adaption, but as well frame design. Looking at the monitor's specs may help to decide.
The most simple would be a VGA adaptor, which just needs to swap some lines. Of course you would need to only used modes fitting the frame timing. There are many descriptions out there, so pick your favorite :))
C64 / Atari 8-bit: not a chance
Sure it's possible, but beside signal line twisting, some additional components might be needed.
Atari ST: this appears like it might be vaguely possible for lower resolutions (same refresh rate horizontally if I'm reading things right) though the monitor port on the ST has a bizarre pin arrangement
Again, just a pinout swap needed - much like ith VGA.
Amiga: again, maybe, due to refresh rate
Same as Atari.
Sharp X68000: don't know much about this one
Before thinking here, get one first .. they are somewhat rare outside Japan :)
Obviously I could just get a Macintosh II or equivalent
Obviously you should. Best would be a Mac LC475 (family member) as not only the screen fits, but it's as well the best option for classic 68h Macs. But best of all, you could plug in a IIe card and have both worlds in one machine.
I assume I'm correct on the C64 being completely incompatible even with a potential adapter, which is a shame as I have a C64 sitting around with no monitor.
As said before, it would work, but only as B&W and with additional effort beyond a simple cable. Considering you seam to want a compact setup, I wouldn't do it, but rather look for one of these cheap Chinese converters able to input S-Video (C64, Atari 800), composite (TI 99/4A) or VGA and output VGA which in turn can be applied to this screen - or a new HDMI based. They are the most versatile solution for old/home computer video.