Green’s Dictionary of Slang

gur n.

In phrases

on gur [? gurrier n.; or ? the gur-cake, a fruit pastry slice popular with poor Dublin children, consumed when truanting]

(Irish) truanting.

1943		G.A. Little Malachi Horan Remembers 87: A boy on gur that was given a candle to hold was shaking with the fright that was on him.
1981		(con. 1920s) P. Crosbie Your Dinner’s Poured Out! 220: out on gur staying away from home.
1987		(con. 1920s) L. Redmond Emerald Square 259: I had resorted to every trick in the bag, even going on gur for two whole weeks.