BEST GAME I EVER PLAYED ON PC
It is totally worth it!
I've had hundreds of hours of fun in sandbox mode building scenes, machines and vehicles. You have thousands of workshop items that add content from other games such as maps, npcs and props. Ranging from minecraft to call of Duty.
You can also play with your friends in multiple game modes. My favorite is prop hunt. It is some sort of hide and seek where you transform in objects from the map.
The downsides from gmod are that the game engine and tush the graphics are quite dated. Also you need that have the textures from counter strike source installed on your computer for most maps to work. You can find these on the Internet or get them by buying and installing cs source.
I recommend buying the orange box and cs source so you have the content from most major valve games.