I think I missed him on my first play and went back and restarted the game because I didn't think there was a way to catch him if you didn't the first time. (And there wasn't, naturally.) He was really cool at first but he's sort of lost his novelty to me after being a Pokemon fan for so long. I used him so many times that if I were to start another save file on G/S/C, I'd probably just defeat him if he was too much trouble for me and just make sure to collect the Dragon Scale to trade for the Exp. Share.
I did like the way shinies looked in G/S/C, though, I have to say it's my favorite of the shiny entrances (the sparkles before a battle). Gyarados is a pretty great Pokemon but by now he's lost his ~*shiny novelty*~ to me, haha. I was obsessed with him when I was younger, though.