Usage of Fast TM

If you want a defender, go ahead and use it on Snorlax.

I wouldn't touch this Gyarados as B/HP performs well on attack. I prefer my B/HP over my DT/HP for attacking gyms but it's just a personal preference. Depends on your play style though.

If you don't have any other Dragonite, maybe use this TM to get DT and wait for another TM to re roll the charge attack.

If gyms in your area are full of grass mons, using on Scizor may be an option too.

It's really up to you and depends on your needs (and what you currently have).

With the options presented, I'd do Scizor. He's not meta-defining, but fun to use with double Bug and lots of resistances. Your Dragonite is going to need a Charge TM, too, so that's a bigger project.

I'm actually thinking about doing the same to my scizor! And I would leave your L/BS as is; I predict that moveset will be nasty in PvP. Plus, with the way the new gym system waters down Pokémon over time, it's not as essential to possess great defensive movesets (in MY opinion). As well, Lick generates energy faster than ZH, at least on offense, which will allow you to fire off those BS's faster.

B/HP ain't shabby on gyarados; I think the ability to dodge more frequently compensates for any cyclical dps or energy gain difference between dragon tail and bite.

I too would change dragonite in the future