(Default: C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod) If the above didn't work and you don't mind losing all your Garry's Mod workshop content. (See 'Downloading' at the bottom of your steam client.)
(If you're having trouble opening console when you launch Gmod, open settings first then hit ~. (Check the very bottom for last resort options.) A. If your game displays this message regarding the map try just unsubscribing and resubscribing to THIS MAP ADDON.
If not you may want to look into running those games or reinstalling them with gmod to try to get them to associate with each other. It will show an X no matter what if you're using a texture pack and that's fine usually, but if you have the games installed they should show up checked. Also while we're at it, check the little controller icon at the bottom right of Gmod's title screen to ensure 'Counter-Strike Source' shows a check mark and not an X by it.
