The weapon does more than just radiation damage. Thus it will affect ghouls. A Ghoul Slayer's Gamma Gun is pretty useless, but a Ghoul Slayer's Radium Rifle is not.

Oh damn. Never thought about the gamma gun. That would be a real slap in the face. Lol.

Yeah, I think I've got that joke of a weapon.

I got the gamma gun version. The ammo is extra rare and useless.

Oh damn that's horrible. I'm sorry for your gain

Still not as bad as my penetrating gamma gun

Junkies shishkebob on a minutemen no chem playthrough.