In DBZ did Gotenks beat Super Buu?

Yes, he did: both in the manga and anime.

Rather easily, in fact, in SSJ3.

he lost to buu when his fused end and buu warpped gotenks up donig there fight and he beat up gotenks he did not beat buu easliy buu was toying with gotenks he told gohan that then pwoer up when him gohan fight