Well, let's start off with the basic questions:
1. Is your game a pirated copy?
2. Have you modified your game content in any way? (Aside from addons)
3. Do you have virus scanning software?
Before you answer, do the following:
Scan your computer for malware/viruses.
Reinstall your Garry's Mod by deleting the "Steam\SteamApps\<Steamname>\garrysmod" folder.
If that doesn't fix the problem, reply by answering the above questions.