I'm sorry, I just can't get over the fact that there's a character that is powered by his confidence. If you call him a weakling and a *** and make him have poor self-esteem, you can kick his ass. I still think that's hilarious.
Kyle kicks his little wussy ass after telling him he's ugly and no girls like him lol.
Kyle. He would know his radiation weakness so that's an easy win. Or he could do something like this.
http://i35.photobucket.com/albums/d184/tgtshazam/LegendsOfTheDCU38p22.jpg
Depends on who is writing EACH character.
One might see Gladiator written as an unstoppable beast, with Kyle only being able to make silly constructs.
Or it might have Gladiator exactly as shown above (unstoppable), but Kyle using his ring as it is supposed to be used (i.e one of the most powerful weapons), and thus defeating Gladiator even though Glads is giving 200%.
Or you might have Gladiator drawn as nothing more than a minor irritant who cannot even keep up with a tidal wave, and Kyle drawn as having powers that almost border on what he was as Ion.
Either character could win, and enough reasons given on WHY they should win.
I'd personally pick Kyle.
Written to his fullest he can beat Gladiator (even when Glad is written to his fullest).