Bud you can try this:
Unsubscribe from your workshop, to avoid confusion with other addons.
Head over to your Garry's Mod folder which should be in DRIVE LETTER/steamapps/common/Garry's Mod
then run the factory reset .bat file (This will delete all the addons etc. clean up your gmod)
Try installing CS:Source if you see black & pink stuffs all over the map along with errors, then try joining the server again.
also try lowering your settings if theyre high and your PC cant handle it, except the model details so the cars are not weird (maybe lower resolution too? hell if I know)
useful commands: r_3dsky 0, r_shadows 0