Gyarados hasn't got a whole lot to fear in Johto really. Just two Magnemites, T-bolt Dragonair and Thunder Dragonite.
Originally Posted by Fat Hemp Man
Basically, GSC is a cake walk. Hell Red is only though because hes over leveled. Any Ground types walls Pikachu, Steel type walls Venusuar, Ghost type walls Snorlax, Dark type walls Espeon, etc.
"Any steel/dark-type" isn't very helpful because you're not very likely to be training one. There's Sneasel, but I don't think it'll do a very good job against Espeon unless you've been saving that Shadow Ball TM.
Venusaur seems to enjoy spamming Solarbeam without using Sunny Day so any flier with Fly can avoid being attacked entirely. Misdreavus is there to trivialise Snorlax if you don't have Gastly.
That just leaves Espeon, Charizard and Blastoise, and they can be pretty tough even with the right resistances. Espeon is probably the toughest one.