Stab = SE
HP hits like neutral on draggy because of stab.
Same goes for OR, it's super effective, but without stab it hits like neutral w/ stab.
You got yourself the best Gyarados, imo.
DT is a better option in general. It's better specially against Dragonite and also Tyranitar, since he resists Dark.
Bite is only optimal against Psychic, Ghost and Steel, three types that you most likely will not see in gyms. Maybe with the gym rework Steelix will get a boost and Bite will become more useful, but that's a big MAYBE.