Spend Magikarp candies now or wait?

Nobody knows what comes in future. It could happen as well, that the dragon-tail-version will become a valuable discontinued....
Or the possibility to reroll moves, or a rebalancing of moves - nobody knows, nobody can tell you.

You play the game now. Look what you need now. Do you need a decent generic attacker, or an additional high-cp-defender?
If not, you could wait, if yes, you could concider evolving one or two of your karps.

Maybe you have got sufficient attackers and defenders. So you could think about investing your dust and candies in maxing out your existing pokemon instead of new evolutions.

What do you do, spend or wait for gym rework?

At the moment, i am content with my attackers and defenders. So i collect some dust and candy, until i feel the urge to do something.

As mentioned, I think you need to look at your own situation and see whether it makes sense to evolve or not based on your needs. As you have 2 already, getting Dragon Tail isn't a big deal IMO since it doesn't have STAB.

I have no Garys here at all, but 700+ candies. Still looking for a higher IV one (91+) to use as a gym defender...

Since they never spawn in my area, I'm content to wait.

Also, as there is a shiny variant currently, it'd probably make sense to save enough to evolve and power up one as well (so 600+ candies) since a high IV rare one would be nice to powerup, even at lvl 1 (at least for me)...

With 900 candies I'd definitely evolve one now as you still gonna have enough candies for another one in case you get a good shiny.

I am in the same situation than you. I ave decided to wait.

I do not need an extra Gyarados now.

It's up to you, but I think I would try to get one of the DT versions. I think it makes a better Gyarados than the one we knew in the past. I evolved 3 recently and got 2 DT which I'm very happy about