Is Garry's Mod good?

Depend of what you want. Base gmod is complete garbage, only usefull profesionnally. That's also why i don't recommand cracking gmod, you will only use your internet for nothing. And even some mods it might not be worth it. Prop hunt is good for 30 minutes, but like sandbox, you won't stay long. But some like darkrp... Oh god i have spend 1000 hours on it. So depends on your taste.