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Gyarados is a bit weak for a special moveset. With only 60 base stat in SPATK, it won't OHKO anything. The above is an physical moveset and Hydro Pump is to use against Rhydons etc.
Gyradose doesn't learn Aerial Ace, replace that option with HP Flying. And Taunt/Thunder Wave would be a better option over Hydro Pump but since it's ingame Hydro Pump should do just fine.
meh
modest
surf
fireblast risky but its youre choice
calls for fillers his move pools kinda crap so either earthquake/dragondance/hyperbeam is nice but the stalls bad
For a special set Gyarados should have something like
Hydro Pump
Ice Beam
Fire Blast
Thunder
HP Flying would be nice somewhere on there but you probably can't get it.
Don't use it though, it's basically used in competitve play for killing most of Gyarados's counters, but ingame it sucks. Stick with the DDer