Green’s Dictionary of Slang

(Irish) a term of abuse.

2001		G. Coughlan Everyday Eng. and Sl. 🌐 Gee-bag (n): general term of abuse.
2008		S.J. Martin Ride On 399: You’re a gee-bag of the highest order, Collins. I’ll bleedin’ get you for that.
2009		C. Kilroy All Names Have Been Changed 78: I heard them laughing with bravado at my retreat, at the great triumph it denoted. ‘Little Trinity gee-bag,’ one of them shouted.