In your Steam library, right click Garry’s Mod and open up “Properties.” A pop up window should open. Choose the “Local Files” tab and in that menu hit “VERIFY INTEGRITY OF GAME CACHE…” Let the process finish completely. Also, go into your Garry’s Mod directory and delete your garrysmod/** folder, and ensure you’re unsubscribed to every addon in the workshop via the Steam workshop menu, rather than ingame before you delete your root folder.
Computer/(C: Drive)/Program Files (x86)/Steam/steamapps/[YOUR USERNAME]/garrysmod/ <-- DELETE THIS ONE
If this doesn’t rectify the problem, a recent engine change could have fucked you over due to your OS and system specs, so I would recommend a much needed upgrade to Win8 or Win7 if you’re afraid of change.