[REAL BEST MOVESET] The Efficient Gyarados.

This answer depends on
Species composition: Obviously if Dragonite/Tyranitar is common, then DT will be superior. Only useful situation for bite is Psychics, Ghosts, which are not seen in gyms often (and often there is a better option). DT is superior for the matchup spread therefore.

However, this is counterbalanced by the composition of defending movesets. You can usually (but not extremely safely) fit 2 dragon tail between a 12 power defender move (e.g. Zen Headbutt)

With bite you have incredibly more flexibility to choose when to dodge and not to dodge.

If the defender has a lower than 12 power move then it is going to fire more frequently and you will definitely not be able to get 2 dragon tail for a dodge.

So it depends really. DT for general coverage and matchup spread, and Bite for flexibility to deal with dodging.

And if you want to ask my personal opinion: I prefer Dragon Tail/Hydro pump. Bite feels more like a scratch to me, it does very little damage, at least DT you can see the enemy HP bar dent

I have 95% IV bite/hydro pump Gyarados. It is my first attacker and I would not change him for any other mon. Bite is weaker but it gives you a big dodge window. I regret that I didn't evolve bite/fire blast arcanine. I was waiting for 95% IV growlithe and niantic have changed the moves. My stupid IV fetish :)