My first Gyarados

I would power it up, then again I have Bite/Outrage Gyarados so my opinion may not be terribly vaild. At least yours has the second best defending moveset.

Congrats on the dex entry and near perfect karp. This is not a bad moveset. Crunch performs surprisingly well on defense in my experience. As for the Pokémon…not worth it. Gyarados isn't the best defender thanks to its very exploitable weakness and cannot out-cp Snorlax, Tyranitar nor Dnite therefore I would say it's not worth the dust atm from such a low level. Maybe if you happen to have excess dust and candy you could put into it?

You'll need to take it to around L25 before it is relevant. Find out how much candy and dust that will take. Once you are willing to part with the candy and dust, make your move if the gym meta still warrants.
You don't know what Niantic will do. Once you are in a position to get a high CP defender and use him, you should do it. Just don't get stuck halfway.

I have 2992cp gary with 78% IV (+14att) same move set and today he sit on gym first time.

I have a DT/C and powered it up to level 31. I think it's an underrated moveset for defense. Gyarados will never be a great defender, just a high CP one. This one is slightly more annoying moveset than the others.

That's what my 100% got, too. I didn't power it up. If it had had Dragon Tail, it would have been a different story.

" It's currently level 3 with 243CP"

Assuming you are not a low level player, powering up from level 3 will take a huge amount of stardust, as well as candy. If you are considering using such huge resources because you expect it to top gyms with it's high IV, note that it will still be well below just about any Tyranitar or Dragonite at a similar level, inlcuding ones with low IVs. If it were me, I'd wait until I caught a good one at a higher level.

400 karp candies on a lvl 3 karp???

You must have ALOT of karp candy?

I would never evolve a karp at any level lower than 20 even for dex entry and even then I am more inclined to wait for a higher lvl one to come along.

Save your candies and wait for a higher lvl imo. DT+CR is a good def moveset btw. I evolved a golden karp and got that moveset... its decent for def.

Crunch is very annoying in gyms. But you don't have to hurry with powering up. I've never evolved a Magikarp below 100 CP, maybe you'll find one high level enough.