He doesn't fight Buuhan, he runs away from him. GT was billed as Toei's own take on it being a sequel to Z but we have no idea if the anime actually directly followed Toei land filler into its continuity or not. Same with how all of Toei's original Z movies follow to the anime and the manga series but generally don't give specific times or places; Movie 8 set ambigiously at some point during the period of the Cell Games's ten day countdown or Movie 9 set between a few months to a year with Future Trunks returning to visit everyone. Erring on the side of caution, we can only assume where GT still lines up when it can with the original manga that Goku ever fought fat Majin Buu, was scared shitless of normal Super Buu, and terrified of Buutenks and Buuhan, witnessed Buff Buu's power (exceeding Super Buu's), and then how it got much weaker in Kid/Pure Buu. Its a stretch to go to Buuhan and Buutenks. Yes.