There are two spreads like this for this version of Gyarados. I'm using simple 252 HP/252 SpD Careful to counter Infernape, Heatran, Kingdra, etc. Also handles quite nicely Scizor and Lucario if neccesary, surviving at least one hit. As a mixed wall it works fine.
Or RaikouLover's 216 HP/112 Def/44 Spd/136 SDef - A bit mixed defensive EVs as it takes better Lucario and Scizor, however Infernape almost kills you with SR up after Nasty Plot with Grass Knot and Life Orb Heatran kills you for sure with HP Electric. Your call, as both spreads works fine ;).