He's not beaten Gohan. Not at all.
The only reason he did good was because of rage plot. Burma got hurt, and somehow he tapped into his love rage and tickled Beerus.
We know for a fact Goku wasn't just sleeping and eating at King's Kai place, he was training his ass off everyday.
If we're considering that Vegeta was more powerful than SSj3, then we need to believe that after the BUU saga, that Vegeta did nothing but train, where as Goku barely trained.
Going back in looking at the BUU Saga, Majin SSJ2 Vegeta with at least x2 power up was almost beaten by SSJ2 Goku. Where has it was hinted that if Goku went SSJ3, he would have made Vegeta his plaything.
SSJ3's boost was far above BUU saga Vegeta, it was ridiculous. This is the same SSJ3 that was useless against Kid BUU, a weaker version of Super Buu, but just more chaotic.