Well, if you throw one in a gym with multiple Gyarados or mostly water type pokemon it might be easier for opponents to take down. On the flip side, Gyarados can be very useful in a gym lacking those types. Regardless, it's a pretty solid mon but I never put one in a gym with a lot of water-types. I prefer gyms with several types over being at the top of the line up.
What do you use your Gyarados for?
I usually just stick him into a gym for the high CP buffer (have a 100 IV one just for this purpose)
I seem to always think that Gyarados is either a bad Vaporeon or a bad Dragonite, but it is probably easier to obtain than a Dragonite so I think that is somewhat justified. However, I really don't see what I can use a Gyarados for if my Dragonite doesn't faint.
As we know Bite is tied with Dragon Breath but:
VS Neutrals: Dragonite has a STAB quick move
VS Water types: Dragonite does not get its charge move resisted
VS Grass types: Dragonite double resists Grass
VS Rock/Ground/Fire: More of an even match here
VS Top Psychic/Ghost types: Super effective cancels out the STAB bonus but Dragonite has higher base stats
VS another Dragonite (assuming Gyarados has Breath/Pulse): Attacking STAB super effective on Dragonite moves cancels out being weak to Dragon, and again Dragonite has higher base stats
Answers
This is a great comment. Gym composition makes a big difference in how hard it is to take down a gym (unless, of course, it is all Lapras or Snorlax).
I know that most folks I know like to be in the back of the gym, so they always put their highest CP mons, regardless of type.
If somebody wants to take down a gym, THEY WILL. The only benefit to being in the back of the gym is if the attacker runs out of revives or runs out of time.
Let me ask all trainers to put some variety to make it harder for attackers - for example, if there are a bunch of Gyarados or Vaporeons, put in an Exeggutor or a Rhydon.
For some Pokemon, you power them up and put them into gyms just because you LIKE them.
One of my friend, an Eevee lover, once planned to max out his 100iv Eevee to go over cp1000 and put her in gyms. That would actually be cool - who gets to see a 1000CP Eevee often? But after the CP Adjustment, Eevee will never make her way over 100CP. So he aborted the plan.
Same goes for Gyarados. A 3000cp+ Gyarados dancing at the top of a gym would make me feel pretty good. I have the exact same situation as you do- hatched that little Magikarp, originally quite sad, but then I fully realized his potentials. Spent 16w Stardust on him since.
It's not always about CP, combat, or coins. It's about fun.
I maxed out 2 w/ HP and I use them all the time. As much as if not more than my HP Vaporeon. They are usually my go to attackers against Snorlax. Sadly I don't have an offensive dragonite. I evolved 2 Dragonites and both of them got steel wing :/ i cri evertime. Finding it hard to bring myself to grind out 125 dratini candies again only to get burned by the moveset.
Even before the CP update my Gyarados was always in Gyms defending them, and he might be crap but once he won a battle all by himself and ever since has 1 Battle Won underneath him.
Mine has B/DP which is his best defensive non-legacy moveset and I know he's easy to kill but I don't really use him for attack since so he's on top of gyms and I'm happy. :)