National Advocate 15 Dec 1931 Gregory goes to the home of his Granny who lives in Howick Street (Bathurst)
Electoral Roll 1936
SHIRLAW Sarah Ann Howick St Bathurst
NSW BDM death
18270 / 1936 SHIRLAW Sarah Ann parents John / Sarah Bathurst.
Because Gregory Ernest JOHNSTONE is so well know in the area I do not think that SHIRLAW was a name assumed for the purpose of avoiding attention.
If he was born to unmarried mother he might have been brought up by another family, or his JOHNSTONE mother married and Gregory assumed her husband's name etc.
Do you see the word "alias", or is it just "known as"?