Why did Freeza power up then?Honestly I'm not sure that Gohan needs to be above 900k. He did beat Freeza down, though Freeza was caught completely off guard. And even considering that, his attacks only hurt Freeza "a little bit."
Goku wins after holding off Gohan's initial burst of rage.
If he could easily defeat Gohan as he was, why did he power up?Because he was angry.