Whittaker did not lose his belt for pulling out of the fight. However, Gastelum believes he should be the middleweight title holder, per MMA Fighting, and even held up a title belt at a press conference:
Whittaker was forced to pull out of another title fight early last year against Luke Rockhold for an undisclosed injury. He has not made a title defense as of yet. One was supposed to occur against Yoel Romero, but the latter fighter did not make weight.
Right now, however, the most important thing is Whittaker's health. UFC President Dana White told reporter Megan Olivi on the UFC 234 preliminary card broadcast (h/t Alexander K. Lee of MMA Fighting) that the hernia "could have been fatal" if it "popped out during the fight." He also noted that Whittaker was "in the recovery room."