Do I have to have a source game downloaded to play?

Do I have to have a source game downloaded to play, or can I just have it in my library? I ask this, because Garry’s Mod won’t start, just hangs at the beginning loading screen indefinitely, and because all of my source games were put on ‘Download paused’. If this is not the case, then what can I do to stop it from hanging?

Had the same problem on my laptop, right click the game go to properties than click set launch options and add “-window” (without the “”) the game will launch windowed but you can chance that in settings atleast the game launches now.

You need at least one source game (TF2 will suffice).