Gyarados is way better in Gen 8
With the ability Moxie, you can dynamax and use Max Airstream and boost your speed, knocking out your opponent in the process. Once Dynamax is over, with boosts in your Attack and Speed, you will completely dominate your opponents team.
Primarina doesn’t have it’s Z Move in Gen 8 so with dynamax, Gyarados is better in my opinion
Hence, for Gen 8, Gyarados is better
Hope I helped