That's what I was thinking. Building a deck around this Gyrados would have 2 issues:
> Getting 3 magikarps into the discard pile w/o Battle Compressor
> Once Gyrados is knocked out, you can't set up a 2nd one because all of your Magikarps are in the discard pile. You could put them back in the deck w/SuperRod, but then you gotta discard them all again for the attack to be effective. Seems like a lot of work to get up to a possible 180 damage, especially with GX's running around @ 240HP and EX's with bands up over 200.
Yeah, the biggest problem will be setting up a continual stream of Gyarados. As others have mentioned Broken Time Space was the reason the Tail Revenge deck worked back in the day. Mind you, at 150HP, it's not inconceivable that the opponent will be limited to 2HKOs on your Gyarados.