Kevins powers are sorta like the master control he can change into anything at will, plus since he use earth species DNA he never mutates into a monster.
He can also go from half animal to perfect form remember when he was trying to frame ben and he turn into perfect copies of his aliens.
Gwen had to rely on time limit rule for a while until she unlocked the omnitrix master control she lost it like Ben did.
But the first day she got it she wrote down all she remembered doing to the watch that unlocked it.
She went on a journey trying to get it back and she never does.
She ounce in frustration tries to hit the watch on the wall at the rust bucket but forgot that's where the window was and they had to put a wood board their for a while.
She does however does learn that trick from alien force where you just put your hand on the symbol and change right on form. (Remember all the quick changes Albedo and Ben did in their fight when they first met.)
She also unlocks Humangasuar, jet ray and big chill on some of her unlock tries.
Sometime later in the future, Kevin accidentally gets hit by doctor animos helmet beam. This evolves his powers that he no longer has the 1/10 rule any more.
Now the animals abilities are at 100 percent. He also doesn't have to absorb animal DNA anymore he now can tap into the genetic code of every animal that has ever lived on earth that includes Dinosaurs.
He now has the powers at full, of all the species that had ever live on earth combine with his matter and energy absorption(By the way he still never goes crazy).
This came at a price, when Kevin tries to absorb Tetrax (that diamond head with the flying boards) powers he realises he can't take any alien powers.
Now it's the of all earth wild life, alive and extinct enhanced with mater absorption and lightning vs the powers of the omnitrix.