Alright, so I think I got it figured out. For some reason, and I don't know why, but my Gmod just didn't download one of the WiltOS addons (the Blade Symphony one, to be specific). I was subbed to it, and it said it got mounted, but no GMA was ever downloaded to any folder. Unsubbing and resubbing 2 or 3 times, forcing to file to download, seems to have fixed this issue.
for reference, the error in the console was like 1 line
WorkshopDL: There is no file to mount for '893244794'!
WorkshopDL: Failed to read addon file for 'SH Whitelist Content' (1251198810)
It looked like this for me (not the right addon, just the first one I grabbed off my console for an example).
This has been fixed for me.