I'm having a MAJOR problem starting up Garry's Mod

So I recently downloaded gmod on steam, and when I tried to start it up, it wouldn’t even launch. I have hl2 and it seems to run perfectly fine. I’ve tried to look up solutions, and I’ve tried everything from uninstalling and reinstalling gmod (4 times and yes i deleted all of the game data) and I even uninstalled hl2. I verified the integrity of both games and restarted my computer. I had bought TF2 but hadn’t even installed it at all yet, so that couldn’t be the problem. I then thought that maybe hl2 was just being an asshole, so I bought the cs source, and uninstalled both hl2 and gmod once again (making sure to delete all of the files) and then I tried to see if I could run gmod under the cs source, and it STILL didn’t work. I don’t know what the heck is wrong with my gmod, but it’s really fucking frustrating because I want to play it. I don’t get any error message, it just says that it’s ‘running’ and then it stops. I know that my specs aren’t the problem because I’m running on a 64-bit alienware with a dedicated CPU and graphics card that can run higher end games without a problem. Some help would really be appreciated.

‘run gmod under the cs source’ - what does this even mean?

You don’t need any other game but gmod to launch gmod. The reason why it’s advised is because gmod uses assets from other source games. So buying CS:Source wouldn’t have done anything.

So how do you launch the game? By clicking play on Gmod in Steam itself?