GMod Can't mount any of the games I own

http://vgy.me/YBDFfp.png

Edit:

Is this related to the fact that I’m family sharing those games?

Are the games currently located in your ~/Steam/steamapps/common folder?

They’re all available, and mountable if I log into the account I’m sharing from, but not mountable if I’m logged into the account that they’re given to.

Then I believe there may be a family sharing restriction.