Where can you find the Gamma Gun and the Atom Cats?
Solution 1:
The Decayed Reactor Site's southern dome contains a dead Child of Atom cultist with a Gamma Gun.
I did some searching, but as far as I can tell, the "wild cats guys" aren't a thing in Fallout 4. Did you mean the Atom Cats? They can be found at the Atom Cats Garage.