It depends on what you do.
If you like to beat level 8+ gyms solo, you need a full 6 powerful lineup. Look over your lineup and decide would a 2nd Hydro Pump Gyrados fit in.
If you need more high CP Gym Bait, Gyrados even twister Gyrados is one of the easiest pokemon to level up to get to 2200+ CP to be on the top half of most gyms.
If you find yourself leveling up a lot of gyms, you might want a spread in CP levels so you can make a decent team at multiple CP points for the Gym your powering up. I tend to aim a little lower than Gyrados here as I have my 1350-1150 "Team"
