Just because Gyarados has low base stat on sptk doesn't really mean it is not viable to be sptker on pvp. But just Atker is the most/best recommendation.
Creativity, Tactic, Experience, Crazy most people who has that could run an Sptk Gyarados. No idea how but I was once deadlocked/checkmated by an Sptk Gyarados. We should sometimes refrain from limiting Magikarp's immeasurable power.