Gyarados Moves

Do you have a B/C one? No STAB on those moves, but might not be terrible if you want to solo a level 3 raid requiring dark moves (Alakazam). At least since he's maxed out.

I am tempted to use a charge TM to try for HP on mine. Gary is fairly useless with the gym rework, but could be a decent attacker with the right moveset

I'm in a similar situation. I currently have B/DP. I'd be happy with B/HP for general raids, DT/O for future Dragonite raids, and I think even B/C for dark raiding into Psychic & Ghost. I think that pair (B/C) will be upgraded for that specific purpose.

I've got several Gyarados that could use better moves, but I never use them for attacking, before they were just gym placers, now I don't even bother doing that. So I'll change their moves once I start having TMs spare. They're a low priority in the current game meta.

With the new gym system and raids, gyarados has less of a place now than ever. I wouldn't waste a TM on him.