Fast TM Gyarados from DT to B?

never

My 5 are all bite, Gyarados isn't great for a lot these days, I at least want it to have fluid motions and be able to dodge

Depends if you have outrage as a charge move. If you do, he's actually good as a dragon counter. If you have crunch, definitely tm it. Hydro pump, I'm on the fence on, but I have done it to one of mine. But I like bite over dragon tail, so that's just me

It's not the best use of a TM, and usually I'd say nah. BUT, if it already has Crunch and you have the TM's to spare, a double-dark Gary will be useful against FB Mewtwo. I just did it to a DT/C Gary (I have others with DT/O, DT/HP and B/HP). No worries.

DT will charge up crunch faster so it is possible to get off an extra crunch and extra damage with DT. If not, bite is better and easier to dodge with. I had one high cp with that moveset and did a fast tm for bite in the end.

I ran Both movesets on pokebattler a few weeks ago. If they have updated m2 stats there, you should be able to run it yourself.
The are pretty close.

I'd rather have crunch than hp but then I'd rather have a t'tar than Gary so there you go...I guess there is an arguement that it will be better versus focused blast. I guess we will see.