I have that one! It was the first gauss rifle I found in game... I hadn't even seen a normal one up to that point. I was lv 51 and found it on a legendary super mutant.
Unfortunately, I'm not too happy with how the gauss rifle behaves in general. The projectile travel speed is low and it seems to have a larger hit box, meaning it's more difficult to thread the needle and you'll often hit fences and such that don't appear to be in the way..and targets can easily decide to move while the projectile is in the air. It has huge recoil, so if you don't finish the job in one shot, you're going to have to wait for the second. I use the guass rifle against a heavy target when I have a clear suprise shot, but then switch to plasma to continue the fight. Of course, one surprise hit from that super guass rifle generally destroys whatever it hits, even legendaries. It's a great opener, and it's good in a fight against one large big target.... but otherwise pretty meh. I often don't bother even carrying it around unless I have a mission that I know will pit me against some big heavy targets. If you want to blow up some power armor though, that is the right gun for the job!
I really wish there was a better 50 cal sniper in the game.