What should I do with this legacy Gyarados?

I personally don't power up anything below 85%IV, I would make an exception with something like this as it has a legacy moveset but if you have a Lapras with a good moveset it will make the job so much better than Gyarados that still does not worth the investment.

I suggest just keep it as a nice trophy ;)

You got one of the rarest pokemons, would that not be a shame if You don't tresor it...

Okay IVs but its double attacks are more important, and You can't get a better one anymore... Just power this beauty up :
;-)

I have one with the same moveset, powered up to the max, after the stat changes it replaced Dewgong as my main dragon killer. I also have Lapras with FB/IB and FB/Blizzard, but the Gyarados is easier to power up because Magikarp candy is easier to get than Lapras candy, and it also feels better when dodging somehow.

I'd power him up and use him. As an attacker the IVs aren't that important.

Thats a great Gyarados, my legacy one has water pump and I would prefer dragon pulse, even with the bad IVs. and yours is great 15 ATK! Like you said, dragon killer!