is bite crunch Gyarados good for psychic

It could be useful for Focus Blast Mewtwo raids. But aside from that I dont think it will help that much. But if you consider the usefulness of his others sets having 1-3 uses aint bad.

Yes, it doesn't get STAB, but it does get the more important SE, and does it from a big attack stat without taking SE in return. If you lack enough powerful Dark counters, he puts in good work.

I have 2 lvl 39 bite/twister Gary, when one day I swim in charged TM's, I will use TM on them, crunch will actually be my preference for them.

Hydro Pump will do more damange on Psychic with hits huge Power Damage. Even tho STAB is lower than SE Hydro Pump is still stronger than crunch

It's good but not amazing vs Ghost / Psychics. Use it as it is. Don't use a TM on a Gyarados, because it's not the best at anything. It's good enough to be used right now but I wouldn't take it to level 40 in the future so I would leave it as it is.

Save your TMs for Pokemon that are more useful such as Machamp, Tyranitar, Dragonite et cetera.

I have 5 bite/crunch Gary's & put all 5 plus a bite/SE Tyra up against Alakazam in a solo raid. I'm level 36 & all the Gary's had decent cp but they were pretty average imo & I failed 1st attempt. Left the highest Gary in, swapped for 2 double dark Houndooms, dark Arcanine & Umbreon, successful 2nd time around. The Houndooms made the biggest difference having STAB plus taking less damage than the Garys