TM Legacy gyrados

Since you have a few good Dark-type attackers, do not TM to Bite/Crunch. WF/HP is best on attack.

Keep DT, and TM it to Crunch, then it will become a great gym defender.

In the era past, electric types were relatively toothless.

Given the double weakness to electric, and the influx of Zapdos/Raikou, I find that Gyarados is often the weakest link in the gym defenders of my region. I often regard it as a free kill and zero threat.

I would either leave it as it is, or TM to WF/HP on the offense depending on how loaded you are with Kyogre. Water offense has no immediate use in the current raid meta. Personally, water does not even make it to my gym lineup.

Keep it! You can use him as a Lati@s counter with those double Dragon moves for the next two months

I actually just used a CTM on an old Gyarados with DT to get him to Outrage just because I wanted a full dragon Gyarados because I wanted one with each moveset. I'd say stay with it just for the variety

Solid snake has a good point, although I'm not sure how effective it would be vs. a Dragon with STAB for the Dragon attacks, even if said dragon is weak to the Boss's dragon attacks. Raids are all about DPS, after all, and getting your damage in, not about avoiding taking damage.

But if you do TM it, Waterfall/Hydro Pump is the way to go. Not quite as good as Kyogre, but doesn't require rare candy to power it up to the high levels.

Double Dark Gyarados isn't that great. However, there is something tickling the back of my mind about a Mega form for Gyarados that gets a Dark typing, and might be really strong. But if that works like an evolution, well, then the current moveset is irrelevant. If not, there's TMs.

Best answer - do what you want with it, but don't throw it away.

Why don't you save that inasmuch as you can't get them any more and create another? Seems really... unwise.