She plays an entirely different role anyway. While Cloyster is best as a physical sweeper, Gorebyss is best as a special sweeper. Cloyster COULD be a special sweeper, but Gorebyss has better sp atk for it. Her only handicap is that she lacks a priority move, but she becomes fast enough where it's usually not a big deal.
It's an all right Pokemon, but one set doesn't make it.
I run Shell Smash, Surf, Ice Beam, HP Grass on Gorebyss. It gives her decent enough coverage, while Ice Beam takes out dragons (as well as flying types and most grass types) and HP Grass takes out bulky waters. As for Ferrothorn, he isn't a problem, because I can lead with a fire type, like Houndoom, which has not only good sp atk and decent speed, but access to strong fire type moves and taunt, and he can resist Power Whip like a boss to boot.