Gmod not detecting CS:S / unable to mount

Before I bought gmod, I had already bought, installed, and played hl2 + episodes, tf2, l4d, portal, and portal 2. I purchased gmod in late September of last year, when gmod 12 was standard and gmod 13 was in the beta. However, I didn’t buy CounterStrike: Source until December. I have it installed and have played a little bit of it. However, when I check the extensions menu of gmod, CS:S is grayed out and the box has a red X through it. I have tried everything possible, including:

uninstalling and reinstalling gmod
uninstalling and reinstalling CS:S
uninstalling and reinstalling both, installing CS:S first
and, as a last resort, completely uninstalling Steam (and all its games) from my computer, reinstalling all games except gmod, playing all those games, then installing gmod
(and btw, each time I reinstalled games, I made sure to play CS:S, restart steam, play CS:S again, and then start up gmod)

Still, every time i go into gmod, CS:S is still grayed out on the extensions menu.
I’ve been getting by by downloading the textures and putting them as an addon (something I wouldn’t normally do, but I feel that since I already own CS:S, I should get the textures even if gmod doesn’t want me to.)

Premature thanks for the help!

Bump?

Another bump? Come on, I need help here.