Is doubtful and up to interpretation to think that Pre Rosat SSJ Gotenks can even defeat Fat Buu. Heck, I even doubt that he can even defeat Good Buu.
Even while having a weakling (in relation to him) like SSJ2 Majin Vegeta absorbed would get him the win. This is without counting Gotenks' poor behaviour in fighting and as well as Vegeta's experience in combat, this Gotenks doesn't even have the Ghost Kamikaze... Buu can basically explode himself and it'd be game over for Gotenks, using the Chocolate beam, or well, torture him to death.