Keep or tm dark-Gyarados?

It depends on your alternatives, but generally I think there are enough superior dark types in the game now. Remember dark types now resist psychic moves even more strongly than they did before, whereas Gyarados doesn't resist them it at all. Look to Tyranitar, Weavile etc.

Dark moves on Gyarados are generally sub-optimal, partly because he lacks STAB. Certainly usable and can competently perform as a dark attacker in the absence of something better, but he's better off with double-water moves. Crunch can provide some coverage in PvP though.

That said, is there really a need for it? If you've managed to raid Groudon already then you probably don't need to spend your TMs. Groudon is only here for 5 more days.

Some examples of pokemon with higher dark DPS than B/C Gyarados: Shiftry, Cacturne, Crawdaunt, Sneasel, Murkrow, A-Muk, Carvanha. Not all of these are actually better attackers due to being too glassy, but it underlines the point of exactly how low Gary's dark DPS is.

If you have access to any actual dark types, Gyarados will never be among the optimal counters, lacking STAB and having a mediocre dark moveset. But like Skywalkered said, Groudon isn't here for that much longer, by this point there may be little reason to spend TMs on making counters for it.

Teachin gyarados a second move is a great idea. I recomend bite/HP as it gives you more coverage especially if you already have a ray, salamance or dragonite on your gym clearing team.

Fast TMs are cheap and guaranteed to flip Bite Gyarados to Waterfall.

Gyarados's second charge move is cheap. Depending on how many charge TMs you have, it might be better to unlock a second charge move. You could roll Hydro Pump with the unlock, or at most have to spend one charge TM.

After that, if you ever decide you want a dark Gyarados again, it is just a single fast TM to get it back.

Whenever we get a pure Psychic Legendary, Bite/Crunch will be useful. The problem is... will it be useful enough to justify keeping?

I'd say no. Gary doesn't get STAB and doesn't resist Psychic. It does slightly less damage (lack of STAB) and takes significantly more damage (no resistance to Psychic), so having a double-dark Gary isn't that useful in raids.

Double-Water Gary, though, is amazing against Groudon. I've got 3, and I use them every time I can.