Failed To Start Game (Unknown Error)

Ok, so I have had this game for about a year, I love it! But last week I started to get this error when I hit play: “Failed To Start Game (Unknown Error)” which then had a steam support link, I tried EVERYTHING on the page, nothing worked. I reinstalled the game, which worked. The next day I come back to my computer, it has the error AGAIN after reinstalling and being able to play play last night! I reinstalled again, it worked. So now every day or 2, I get this error and keep having to reinstall GMod, can someone help?

I had a problem similar to this where the game kept on crashing when I started it. How I fixed was by uninstalling Steam entirely and deleting all the folders for it. I know it may not be the solution you are looking for, but that is what worked for me.

If I reinstalled steam, would I have to reinstall ALL of my games, and also, if I delete all of my gmod folders, can I backup my addons. And this error is only going on with gmod.

I know you would have to reinstall all your games, but as long as you are subscribed to the addons in the workshop, they will download automatically as soon as you run gmod again.

I have like 40 games… so yeah, I might reinstall it when I have a bunch of free time.
The annoying thing is, I have been playing with my sister, and every day I hold us up trying to fix gmod.