Gyarados advantages:
-While both combatants can increase their statistics, Gyarados should take it in that category due to not only being able to stack the effects of its stat boosts on top of one another, but being able to lower that of its opponent's as well.
-Much better versatility that is bound to catch Sportacus off guard, as Sportacus has little to no experience dealing with giant water serpents with fire/ice/energy breath, weather manipulation, wind manipulation, and the ability to inflict status conditions.
-While Gyarados doesn't have the arms to protect it in CQC, it's ability, "Tenacity", which causes opponents to flinch upon contact, should cover for that as well.
Sportacus advantages:
-Has vastly superior range by virtue of hitting an apple to the moon with a raquet, and can fly on his Sky Chaser Scooter.
-Likely has the stamina to keep up with Gyarados.
-His superior lifting strength means that he could grab Gyarados by the tail, as a great pyramid is likely much heavier than 518 lbs if you ask me, and Gyarados would be at his mercy.
The verdict:
I think Sportacus takes it with high difficulty. While Gyarados will likely have an immense AP advantage in the long run and its versatility is more than enough to keep Sportacus on his toes, I think his stamina would be able pull through eventually, as sooner or later Gyarados will resort to using Hyper Beam at some point, and will tire out, leaving him open for Sportacus to keep wailing on him until he wins, if not just grab it by the tail and impale it on a skyscraper or something.
Sportacus wins with high difficulty.