Jolly Gyarados is well suited for competitive. Especially with 6 perfect IVs. You have a perfect Magikarp! It's very valuable without being shiny. As a shiny perfect Magikarp, it is extremely valuable. I really doubt there's anyone out there that prefers a perfect Magikarp over a perfect Gyarados, so I'd say it's safe to evolve without losing any of its trade value. And of course you can use it in competitive too! Congrats!