Using a Pokemon Blue cartridge.
Trying to play 25-30 league, my other pokemon in that level range show up but no Gyarados. He's level 28.
Does Gyarados not exist in Stadium?
According to http://en.wikipedia.org/wiki/Pok%C3%A9mon_Stadium, the Petit cup is more restrictive than just level:
Not only that, but it turns out you can't even use evolved pokemon. Gyrados breaks more than one rule. I think you'll find many of your pokemon aren't selectable for this event.
The rules for the Petit Cup can be seen here: http://faqs.ign.com/articles/387/387658p1.html