Garry's Mod worth getting it?

So, recently I put some money in my wallet for TF2 keys (to buy taunts and cosmetics, etc.), and while I was watching game prices in scrap.tf it says that GMOD is just a key (which is like 4-5 times cheaper than if I buy it from Steam).
My friend has GMOD and other one says that he might get it too.
Does it really needs friends to play or I can play in servers too?
I'm looking for games to have fun, like the modes that GMOD has.
Or is it better to keep the key, buy Brotherhood of Arms and The High Five taunt?
Because a key is 16 refined, Brotherhood of Arms is 9 refined and 7 refined lefts.
The High Five taunt is 7.66 (which means 7 refined and 6 scraps or 7 refined and 2 reclaimed).
So, I have a reclaimed and 1 scrap, and through the time I play when I get 4 weapon drops (which makes 2 scrap combined, 2 weapons needed per scrap) I'll make it to 2 reclaimed and I'll have the price for the TF2 things.
But does I get GMOD instead or these things?
Does GMOD is fun in servers or better with friends?

 

GMod is fun.

Even more fun in a good server.

Most amount of fun if you get a group of friends. Endless opportunities, endless building.

Worth it.