I made all the callculations on example of lvl 40 Gyarados vs lvl 40 perfect IVs Blissey to determine true DPS. I callculated it for every non legacy Gyarados moveset with all possible attack IVs.
Link to a graphical presentation: https://youtu.be/GCGx79SKh0s
Point is that Bite/Hydro Pump gives most possible true DPS (like most of the time, only attacking dragon type defender makes Dragon Tail better).
Bite is only better from Dragon Tail if attack IV is higher than 6.
Gyarados achieves its highest true DPS already at IV attack value of 13, so there is no point in aiming for higher value unless for CP placement in gyms.