Difficult to find him as his name is not unusual. For example, there are three persons of the same name buried/cremated at a large crematorium/cemetery in the Melbourne metropolitan area.
https://smct.org.au/deceased-searchRobert James Gibson Springvale Botanical Cemetery Cremated 19 November 1974; no DoB recorded
Robert James Gibson Springvale Botanical Cemetery Cremated 30 November 1989 - DoB 1907 so not your man
Robert James Gibson Springvale Botanical Cemetery Cremated 25 September 1991 - no DoB recorded
And another at the Melbourne Crematorium but his birth date is 1928 so not your man.
I don't have access but someone else may be able to look up the 1991 death notice, if there is one, but it still may not be your man.
Judith