I've searched almost everywhere but I can't find them.
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.
The places to find gamma guns are mostly at spots where the radiation symbols are on the maps with exception to a few. These guns are deadly unless you have a hazmat suit or a good supply of rad-x. Atom cats are on a location east of diamond city and its symbol is of the junkyard symbols.
I've got three or four gamma guns which were all scavenged from random encounters in the wastelands. I don't remember exactly but they probably came from encounters with Children of the Atom, which I've had happen in a few places where you often get gangs of NPCs spawning, such as the ruined building north of Mass Gravel & Sand, and the crashed plane near the General Atomic factory.
I found mine at a raider camp not sure were but I killed several ants then a legendary Razer somthing came out no were killed it and it gave me the gamma gun.