What is DT/HP Gary good for? Lol (first female one btw)

DT/HP is great offensively, and also very good defensively. Grats! and enjoy your new mons :)

Dragon Tail is the perfect quick move because it's not too slow for offense, and it's got great base damage / energy for defensive AI use. HP and Outrage both have their pros and cons for defensive use, but they are both very good.

Power up, or Jolteons may kill your Gyarados before the release of HP.

I'd power that guy up in a heartbeat and would use him as a defender. All 3 my Gyarados have Bite. That's some good stuff you got there.

I also have a DT/HP gyarados with 89% IVs . I powered it up all the way. It is a great moveset for offense and defense. Hydro pump attacks so fast in the gym defense AI now, I miss my dodge half the time. It may only get used once, but for me it is incredibly hard to dodge.