Is it worth to invest on this Gyarados?

Personally, I would power him up as a defender. In my opinion, this move set is better on defense than any of the earlier moveset options which is about 90% of the Gary's currently sitting in gyms. Once you reach a higher level you will also be happy for the extra cp. congratulations, those are great stats.

Most of gyarados' value is to sit high in gyms. Moveset is less important, as they all fold to electric. It is unlikely that you will find an equally good magical any time soon. Power him up and stick him above a Rhydon :-)

Not sure how easy it is for you to get more Magikarp candies.. If not, its best you power it up ! Its still a decent attacker with great IV's and you will end up with a high CP Gyarados.

Nice one you have there. If you want to power up a Gyra this candidate is as good as any.

I would however be worried about your current level and stardust count because you don't have much to work with. Even if you max out your stardust on this guy you will not reach 2500/2600 CP. Is this ok for you, or do you have better options to spend your stardust on, for example a dragonite or Snorlax?

No....that's really a waste of stardust. I would either invest in another pokemon or just wait till I get dragontail + crunch (decent as defender, it bites jolteon well).