Hi!
According to
this MSO thread, you should be fine to defer JCU med and still be eligible to apply for Monash - Monash will not lose your ATAR score. Someone said they have seen Monash specifically having ATARs ‘expire’ after 2 years, but since your gap year is just one year I don’t think there will be any problem. The only thing you have to consider is whether you will personally find the gap year beneficial - obviously, a good UCAT score is not ‘guaranteed’ and it would be a better idea to find something else to do such as work or volunteer if you were to do a gap year. It’s really up to you!
The only thing to consider from what it looks like is actually deferring - this thread was written in 2011 so it may be different now, but you might actually have to write to JCU explaining your reasons for deferral. They may have policies surrounding deferral that would mean you might have to contact the uni directly to clarify
I hope this was somewhat helpful!