Green’s Dictionary of Slang

1. (Aus./US) the morning [abbr. good morning].

1896		Ade Artie (1963) 45: I got a set o’ coppers on me this g.m. that’d heat a four-room flat.
1906		Gadfly (Adelaide) 28 Mar. 9/2: I was goin’ up ’Igh Street at three g.m. yesterd’y, yer Wershup, when I seen the pris’ner layin’ on ’is back on the footpath outside Central Auction Mart.
1912		Ade Knocking the Neighbors 87: Once there was an undivorced Couple that would get up every G.M. and put on the five-ounce Mitts and wait for the Sound of the Gong.
1920		Ade Hand-made Fables 36: This thing of squaring up every G.M. to the same old Lay-Out.

2. (UK black/gang) a fellow gang member.

2018-20

T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 GM– (fellow) gang member.