Mr. G is a transgenderHe says that he was known as "Helen" when he was at school and said that his balls used to be "inside of him".
The three characters' stories take place years apart from each other.They never, ever meet. The only character who meets Jonah, Mr. G and Ja'mie is the headmistress, and if we assume that all three stories take place simultaneously it means that the formal was happening at the same time as Mr. G's play. Plus Mr. G should be present in the assembly when Ja'mie gives her speech, and in the staff room while she organises the formal. Chronologically speaking, it would appear that Jonah's story takes place first - he spends his time at the school before getting expelled. Then Ja'mie joins the school for the exchange program and leaves at the end of one term. Then, after Ja'mie leaves, Mr G joins Summer Heights High, spends a few years getting putting on plays, then finally puts on Mr. G: The Musical.