Well weren't the Kai's and the Supreme Kai's essentially gods, even though it was never really stated, though that's the impression you got when Piccolo and the other's referrred to them? Well they were just guardians of the universe but the implication. . .
And most of the Saiyans were already far more powerful than them. Kinda like Marvel 'verse, where Hulk is more powerful than the Asgardian gods. . . And most other fictional tales where mortals are always whipping the gods butt's. . .