so why doesn't Gohan..

go Super Saiyan when he's fighting Buu (after Buu absorbs Gotenks and Piccolo) ?

Goku is back on earth now to fight Buu but it struck me as odd that Gohan never changed, sure he got his powers awakened by the Old Kai but still. you'd think that when he feels he can't win, he'd power up and beat Buu's ass in Super Saiyan mode, but he didn't even try and got beaten up pretty bad and finally absorbed by Buu.

Is there maybe something I'm missing? or is it just some oddity in the series?

His Ultimate form replaced his SSJ forms. Old Kaioshin asked him to do that SSJ thing and instead he got an all new (permanent) power-up.

oh I see, yeah I guess I missed that then.

that does make sense, thanks for clearing that up.