Next time, please post in the correct subforum! Help and support is right above all the topics.
Now, there are a couple ways you could go about fixing it, and we’ll start with the least damaging first. What you want to do is right click GMod in steam, and go to “Properties” in the little menu that comes up. Now, in the new window that comes up, press “Local Files”, and then click “Verify integrity of Game Cache” This’ll take a minute or two. When it’s finished, try starting GMod again.
If that doesn’t work, it might be a problem with steam. Close steam down, navigate to C:\Program Files\steam (or where you installed steam), and rename “clientregistry.blob” to something different, like “oldblob.blob”. Now restart steam, let it update, and try to start GMod again.
Third option, do this if neither one of those don’t work. This will wipe all of your custom bindings and video settings, so be wary! Right click GMod inside of Steam, and go to Properties again. This time, stay on the “General” tab, and click on “Set Launch Options…” Now, add -autoconfig inside of the text box that comes up, and press OK. Try starting GMod now.
If worst comes to worst, and it still won’t start, you might have to delete your GMod folder, which means you’ll lose ALL of your addons, if you don’t back them up. Navigate to C:\Program Files\Steam\Steamapps\Put your username here\garrysmod and delete the folder named “garrysmod” inside of that. Now go start GMod again.
This is the LAST RESORT, only do this if it absolutely WILL NOT START. Right click GMod and select “Delete Local Content”. This will make you re-download GMod, and it might work.