Should I buy Garry's Mod?

As others have recommended, I'd suggest checking out some videos online to get a feel for what it is. I got it a long time ago, possibly during a steam sale, and I have had a lot of fun in it. The limits are basically only your own imagination! If you dig around there are some cool add-ons for it too. One of my favorite ones was an addon that allowed you to load in videos and place them in the world, and then watch them.

As stated by others here too, there are so many things other people have made that you can download and play, so it's kinda like a game that can play other games!