Huntail Gets Dark Physical moves while Gorebyss gets Psychic Special moves. The only statistical difference is that Huntail has more Attack, while Gorebyss has more Special Attack. I'd say that a lack of Psychic presence on your current team and being your best specially offensive Pokémon, Gorebyss would be better by far. It can also lean Ice moves, which are pretty helpful, and can get Shadow Ball for coverage. :P
You could also get it Shell Smash from Clampearl to make it even more powerful. It should probably know Surf, as the move is useful and is a good Special STAB move. :P
Just one thing: Gorebyss is a little slow, and being in the Erratic EXP. group may hinder it's effectiveness near the beginning of the game; however, it will become very easy to Level Up later and can be a very strong late game Pokémon. :P
Hope I Helped!