Dark gyarados

You mean a shiny (red) gyarados? They look nice in gyms (I have 3 of them), but are no better or more powerful than a regular gyarados.

I think it would work okay, especially since Gyarados has a high attack stat which would compensate for not recieving STAB bonus. Gyarados also resists Focus Blast in case Gengar has that move. Tyranitar would work better but Gyarados is definitely an alternative.