This may also fix random crashes, this can't hurt anything so try it.

Gmod crashes when you press the console key.

Now open the Steam client>Library>Right click Garry's Mod>Properties>Local Files>'Verify Integrity of Game Cache' Anti-Viruses will mess up your game cache, so have them running while you do this next step! You have to re-verify your game cache, first step close gmod. Gmod Crashes on start-up, or other problems you can't fix. Everyone should do this first fix below, even if your Gmod works. Please go to this link below, I no longer play gmod but the people behind this post are active in gmod. Just stop posting, please keep this read-only. Plus I figure it has to be easy to find on search engines since people keep finding it. I keep this up because I know it will help people out. I will not help you, I have not played gmod in years.