Low cost Seaking is easy to get and can use Ice-type move, Icy Wind. It evolves from Goldeen with 50 Candies.
Effective ice-type move against to Dragonite Although Ice-type moves are very effective against to Dragonite, Ice-type Pokemon is not easy to find and get. Therefore, you may bring Seaking into gym battles to defeat Dragonite.
Reducing damage of dragon-type moves Magneton is a steel-type Pokemon and cuts damage of Gyrados's dragon type moves such as Dragon Breath and Dragon Pulse.
Super effective electric-type moves While reducing damage of Gyarados's moves, Magneton's electric-type moves are super effective against it. Thunder Shock and Discharge is the best moveset for Magneton to beat Gyarados.