The 1947-1959 gap which is the default answer for this genre of questions. Gibbons could also been a alphabet champ in the 4 belt era if we look at the easier side of the spectrum. He'd be able to beat John Ruiz or a Maskaev. Hes way better then Stiverne so if the WBC chose to give him a title shot against Arreola he could do that as well.