Which moveset is more preferable on Gary DT/HP vs DT/O

Outrage is super weird in my experience. I'd rather use Hydropump. I can dodge after using. Outrage isn't bad but if you want to use it safely you've got to know when and what to use it on

hydropump for general purpose use DT/DO as an adequate dragonite counter. neither are worth while considering in defense.

For attacking its obvious DT/H has the highest DPS than any other move set. I'll give DB/H or bite/H the second best attacking move due to availability to dodge more effectively vs DT.

for defending its debatable but anything with Dragon tail is a good defending move.

Both DT/O and DT/HP are great for attacking and which you prefer depends on which defender you are using it against and playstyle with HP being preferable for me personally.

For defending choose the DT Gyarados with the highest CP you have, it's that simple, LOL. A case can be made for each DT moveset having its own merits against its usual/likely challengers.

O gets dodge easily, HP after the move rebalanced became even better.

Hydro Pump > Outrage

They're both great moves. When I'm gonna evolve a Gary, all I ask for is DT, cause all charge moves are great and not easy to dodge. I could say HP is better on attack and O on defense, but the difference is small.
Just maxed out a 97% DT/O and can't wait to sit it on a gym.

They are very close. Each got pros and cons. And depends on the defender if you're attacking, and depends on the attacker if you're defending.