Dragon tail / Twister Gyarados

DEFENSE? What the hell is that?

Just as long as once defence becomes important again you don't put him in gyms, ok:) But for now who is in doesn't even matter.

Not really. Defense isn't incentivized right now and even if it was, gary would suck at it; jolteon absolutely vaporizes gary, even harder than before the rework...

If you're at the point of using TM's to make better defenders, at least use them on mons that would actually make good defenders.

There's a good chance you have better defenders. Battling DT/T Gyarados could be a bit annoying, but in reality, it shouldn't pose any kind of a threat. Gyarados with its double weakness to electric Pokémon is just to easy to knock out.
If by any chance someone would attack your DT/T Gary with a Dragonite (or any dragon type Pokémon), it wouldn't be a bad defensive moveset. But considering all, my advice is, if you don't have a better way to use TMs, just save them for later.

Twister is still a terrible move. I have a 98% B/Twr Gary also. My plan is to use a charge TM. If it gets crunch or hydropump, I will leave it. If it gets outrage, I'll use a fast TM for DT. Anything without twister will be better than anything with twister.