Is garry's mod a scam?

Unless someone is trying to charge you for it, than no.

Garry's mod was a modification program released for free that allowed you to play with a physics engine outside of a game.

Since this was originally released free, how could that possibly be a scam? The entire point of a scam is that someone is trying to trick you into giving them something of value in return.

Now if someone has created a scam that happens to use Garry's mod that's an entirely different thing. But the mod itself is just a mod.