Sonic Boom
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Old Kai's power-up only let Gohan tap into his power all at once without the need for a transformation and the effects it has on his stamina. It's reasonable to believe that Gohan got rusty in the six years since Majin Buu and that he can't access his Ultimate Gohan state as easily as he used to, leading to him just accessing Super Saiyan, which, for a couple of Freeza grunts, is all he really needs.
As for Vegeta being stronger than Gohan, Vegeta's kept up his training. Gohan hasn't. Vegeta surpassed SSJ2 Gohan in strength during the time between Cell and Buu, and never slacked off, so he could very well pass Ultimate Gohan with his constant training and Gohan's refusal to train.
As for Vegeta being stronger than Gohan, Vegeta's kept up his training. Gohan hasn't. Vegeta surpassed SSJ2 Gohan in strength during the time between Cell and Buu, and never slacked off, so he could very well pass Ultimate Gohan with his constant training and Gohan's refusal to train.