err... that doesn't tell me anything. Restart steam, make sure gmod is installed in the right hard drive... make sure your on the right steam account...? If all else fails reinstall again and see if it helps, or get an earlier version.
Okay, I had the same problem when i installed, i simply made sure that Half Life 2, HL2DM and Counterstrike Source All Worked Which They Did... Reinstalld G-Mod. Then Restarted Steam.