the theory is that the target needs to be in a bad mental state
Dorry got his arm blown off before gunko transformed him. And Brogy and others witnessed that before they got transformed.
So, we been speculating that Rocks will get turned after he sees his wife die. With rocks beating garling this chapter, there's a possibility that garling goes for revenge by killing Rocks' wife. Anyway, seeing that should make Rocks go into a catastrophic mental state. Imu will try to Domi reverse him. He will get turned. But due to his sheer rage, imu will not be able to control him and he will go berserk. Forcing garp, roger etc to team up to beat him.
That's my theory at least.