Charge TM gyarados-bitecrunch

I honestly wouldn't bother with Gyarados at all. He's not a good defender and there's nearly always a better attacker option. Save your TMs for something that matters.

Bite crunch is next best thing to tyranitar as dark type attacker. Use it on raids if you need dark dps

if you really like gyarados, then do it!

otherwise, attacking and defending meta is basically obsolete. You only want double stab moves for attacking raid bosses now. gyarados cannot do that.