Green’s Dictionary of Slang

(Irish) a euph. for Jesus! (pron. ‘Jaysus!’).

1973		H. Leonard Da (1981) Act I: da (the standanrd answer): We won’t do a shaggin’ hand’s turn. charlie (awe and delight): Gawny!
1981		(con. c.1920) P. Crosbie Your Dinner’s Poured Out! 63: Gawney, will you look at them; aren’t they lovely?
1982		E. Mac Thomáis Janey Mack, Me Shirt is Black 132: The words ‘Gawney Mack’ is the swanky way of saying ‘Janey Mack’.