It's now 2023 and I seem to have fixed my Garry's Mod to the point where it hasn't crashed once for me, so far. (Although some servers have been absolutely awful to play on, getting 2fps, thought these are the servers with 5+ gigabytes of downloads. Servers that label themselves as "small download" yet still have 1 gigabyte or more, beyond frustrating, maybe it was better when I couldn't play it...)
Anyway, the fix. I did 4 different steps, without testing in between, so I'm not actually sure which ones are necessary, but if you want to figure that out you can.