Whittaker (20-4) weighed in at the 185-pound limit. Gastelum (15-3) came in a pound lighter at 184.
Fighting out of Sydney, Whittaker is the UFC’s first titleholder from Australia. He is seeking his second defense and first on home soil. Gastelum, of California, is riding a two-fight win streak, including a split-decision victory over Ronaldo “Jacare” Souza.
In the co-main event, former middleweight champion Anderson Silva (34-8) weighed in at 186 pounds for his non-title fight against Israel Adesanya (15-0). The winner of that matchup is expected to receive the next shot at the 185-pound title. Adesanya, 29, also made weight at 186 pounds.
Silva, 42, was emotional after making weight, openly crying on stage. Saturday will mark his first fight since February 2017, when he scored a unanimous-decision victory over Derek Brunson.