Garrysmod can do a lot.
Garrysmod as a Sandbox:
Garrysmod can be a simple sandbox in which you throw around objects, create funny scenes or build contraptions by yourself or with friends/strangers. This can range from tires attached to a bathtub all the way up to walking machines, airships or something else that’s just weird.
You’re not limited to just the objects from your games, content from other games or genres routinely ends up on the workshop. From weapons from game franchises to pony models to prebuilt cars and other vehicles, there’s plenty of scripted stuff to dick around with in singleplayer or multiplayer.
Garrysmod as a collection of minigames
Garrysmod gives you access to a few popular gamemodes such as DarkRP, Murder, Trouble in Terrorist Town, Spacebuild and other ones (there’s an emulation of Q3A and UT99 out there, even!) If you’re bored with sandbox, you can join a non-sandbox server and experience completely different games hidden within Garrysmod.
Garrysmod as a game maker
The end result of this is that garrysmod is amazingly flexible and can teach you a great deal about scripting and crafting your own gaming experience. It works as a tool to teach you about programming while giving a lot of options to mess around with.
That’s my pitch, at least. It’s fun, and can give you way more entertainment and value than the $10 price tag may promise.