Adding to
dotwingz's reply, the main reason I have suggested a few students who already got a high ATAR (~99.5) to take a gap year is to keep them eligible next year for med schools that don't accept uni-student applicants - like UQ, Griffith, Monash, Adelaide, UWA, Curtin.
If you don't care about applying to these schools you can start an undergrad degree and apply as a non-standard to UNSW, JMP, WSU without needing to "protect" your ATAR.
JCU would be a disadvantage due to fewer places available to non-standards and if you get a GPA worse than 99 equivalent. But JCU alone is not enough reason to take a gap year.