Gyatso is good, but his team has an incredible disadvantage with Meelo. Couple that with Tenzin and Aang knowing more moves (Tenzin's wheel for example) and the fact that the avatar is inherently more powerful with the elements than a normal bender even without the Avatar state, and you have a major curbstomp.
Ah, nice battle. Appreciate the love for the Airbenders! But unfortunately, as epic as Meelo is, he is the downfall for Team One. He is vastly inexperienced, lacks the focus and training that both Tenzin and Aang have and despite his occasional spurt of power, he is overshadowed by all three of the other combatants.