That's a strange one, I've never heard of a health fund not covering gastroscopies. It's day surgery and you are considered to be an admitted patient and as such you should be covered. I'd double check if I were you. In any case Medicare should cover some of the doctors fee and the anaesthetists fee even if you aren't covered with your PHI. a friend of mine has them done with no PHI and I think she is is out of pocket about $1000 but that includes her pathology costs because she always has biopsies taken as well. I could be wrong but I think that's about the figure she mentioned to me.
There is a legal obligation for doctors to provide you with a written estimate of fees before any procedure so you can give informed financial consent. If you aren't covered for PHI insist on this written estimate of fees before you agree to the procedure.