There is a naturalisation record at the NAA for a Johan Joseph GREGOR. You can make a request for it to be digitised for a small fee. It should give his date of arrival and the name of the ship. There is no way of knowing if it is your man without looking at the record.
http://www.naa.gov.au/collection/search/index.aspxExact spelling on the record is Johan Joseph GREGOR.
Debra