KleerAsDayMay 10Counter-StrikeGameplayi think i forgot to install Garry's Mod (jokes aside, what the fuck happened to my copy???)
Verify authenticity of game files
Your Counter-Strike: Source is missing Counter-Strike: Source
That's the problem. You forgot to install Garry's Mod.
Turns out that I had actually loaded the Interloper version of Dust 2 instead of the real one. Woops!