I prefer HP on an attacking Gyarados because of the current meta. Water is effective against Rhydon and Tyranitar. For Dragons I'm using an ice mon and never Gyarados. For defense I would prefer Dragon moves. For Blissey in particular I don't use Gyarados but hey you gotta use what you have I guess.
GamePress
Gyarados with Bite/HP
I have a wonder Gyarados with Bite/HP (two, actually, but the other is now in a gym), another with DT/OR, around 200K stardust and 350 Magikarp candy.
There has been much controversy over his placement in the Attacker Tier List, but I also noticed that in the Best Ways to Beat Down Blissey chart, he does much better than Vaporeon.
My anti-Blissey squad currently comprises a Machamp with Counter/DP, an Exeggutor with Conf/Solar and a Vaporeon with WG/HP, but needs backup.
Unfortunately, my four Dragonites all have SW (I do use the one with OR), my only Alakazam (level 20) has DG, my six Poliwraths all lack DP, Heracross is not available here and Tyranitar is a distant dream. I only use my Rhydon with MS/SE when I do not need it for defence.
Is it worth investing 17K stardust to add my Gyarados with Bite/HP as backup to my anti-Blissey squad? Are there any other matchups where he does well (even if not optimally; I may lack the optimal attacker/prestiger)? Another candidate would be my non-wonder Flareon with FS/OH, which would require 28K stardust (I already have a wonder Arcanine and a wonder Charizard for destroying Exeggutors).
Report
Answers
Before Blissey towers became common, I would just use Vaporeon or Venusaur to deal with Rhydon. However, when Rhydon and Dragonite/Gyarados are protected by multiple Blisseys, I cannot afford to spend an attacker/prestiger slot on a Venusaur.
Bite is NVE into Tyranitar (which makes its lack of STAB even worse).
I think it's worth investing in. 17k isn't that much dust and if it means being able to take down gyms stacked with Blisseys in one run for that bonus prestige loss, personally I think the investment is worth the time saved.
They perform decent against Rhydon too, but still not as good as Vaporeon I think. Flying typing resists both quick moves, plus 2 out of 3 charge moves, but stone edge is super effective. If your 6 attacker slots are limited you might not have room for Vaporeon so a Gyarados can work fine and also be used against Blissey.
Gyarados is useful in more situations that aren't Blissey, but if having a strong attacker into Blissey is the priority, then I think the Flareon is better. It's fine if the IVs aren't amazing, since you're using it to attack, the CP difference between it and a perfect one is irrelevant.
I agree that its definitely worth investing in - 17k dust is a half days work at most (hatch a few eggs, catch a few mons, cash out on a few gyms). Its an excellent backup for attacking into Blissey - much (much!) safer than Flareon and about as fast too as anything other than a Fire Spin/Overheat version. Its always a great 'go to' to add as a 6th attacking team member, because its a good generalist attacker. And as a bonus, it makes a good defender because when well powered up it will have the CP to sit above most Blissey in typical gyms, and with Hydropump being very difficult to dodge/see coming its not just a high CP defender, it adds genuine strength to a gym (as do most movesets on Gyarados) despite the obvious vulnerability to electric types.
