I own CS:S, yet GMod doesn't recognize the maps

I’ve owned both GMod and CS:S for a while. Most of the content works as intended. Ever since buying CS:S, I see models and textures rather than ERRORs. The only thing not working properly is the maps. I often try joining servers only to get an error which says I’m missing de_dust or office or something. How can I fix this?

Are the games mounted?

Start up CS:S.

Start up the game once, my CS:S had to convert the old files into new files or something which made them not load in Gmod