Once he hears about Zangetsu's proposal, Gabriel is going to push hard for that. He will argue that time and change is absolutely necessary for evolution and becoming a better, happier person. If anyone argues that, he will give (probably graphic and disturbing) examples from his own life.

But he will also say these kids have been through a lot, so if they need a 'break' for awhile, they should be allowed to have that. That would also give the Boy what he wanted- a way to protect these children from the worst parts of chaos that have been inflicted on them.

Gabriel wants the children to be able to decide individually for themselves if they want to want to grow up, and to be able to make that decision later if they want. If the Boy and the Crone don't agree to that, he will continue to argue for it- or at least a day every so often where the kids decide if they want to grow up or not. But he will grudgingly accept them all having to make the decision now, if that's the only option.

Edited to add: Gabriel would also argue that the choice to grow older and the choice to leave the island should be separate allowed decisions. Maybe Royce's idea of having a section of the island where time moves normally would work well with that?