Yes sorry, Straight From Bulbapedia
HeartGold and SoulSilver, the red Gyarados is found in the Lake of Rage. The player has one chance to catch it and the incident triggers the battle against Team Rocket in the Team Rocket HQ in Mahogany Town. When captured or defeated, the red Gyarados drops a Red Scale. While this is the only chance to catch the Red Gyarados in the original games, it will respawn in the remakes after the player defeats Lance and the Elite Four, should the player fail to capture it.
I love Weird things ¯(°_o)/¯