should i get garrys mod

You could wait for it to go on sale and then pick it up for like $1.

If you aren't into RP, then I don't think I can recommend it because those are the most popular game modes at the moment (of course you can play something like prop-hunt, but why pay $9 just for prop hunt?)

I'd wait for a Steam sale and buy it then if you are on the fence about it.