Gastelum was scheduled to face off against the Australian at the Rod Laver Arena in Melbourne on Saturday night.
The fight was the headline bout of the evening, after the champion suffered a freak injury.
Whittaker started complaining of pain in his abdomen on Friday night after the pair weighted in, and he was taken to hospital.
It was deemed that he had suffered a hernia and will be forced to undergo surgery.
However, despite the bout not taking place, Gastelum appeared at the Rod Laver Arena with a UFC world championship belt, claiming that he was now the champion.
He declared that because he had turned up to fight, he had earned the right to be called the champion.
“This is my belt, I earned this,” he said.
“I showed up, traveled thousands and thousands of miles from home, made the weight.
“In my world, the wrestling world, if the guy shows up and makes the weight, for some reason cancels the bout, the guy forfeits the match — I win.
“I am the champion.
“Hopefully Rob heals up great.
“I wish him a speedy recovery and when he heals up maybe he can get a crack at my title.”
UFC Dana White didn’t comment on Gastelum’s claims, but he did give a frightening admission over Whittaker’s injury.
“This was a freak, freak injury,” he said.
“It’s one of those situations where if it had popped out during the fight it could have been fatal for him.”
Israel Adesanya’s bout with MMA legend Anderson Silva was promoted to the headline slot at UFC 234.
White had previously stated that the winner of that fight would be made mandatory challenger to the middleweight title.