Green’s Dictionary of Slang

1. a prostitute, a promiscuous woman.

1929		(con. 1916) F. Manning Her Privates We (1986) 153: Girls o’ seventeen painted worse nor any Gerties I’d ever knowed.
1982		E. Mac Thomáis Janey Mack, Me Shirt is Black 133: We went to Sandymount Strand [...] every Sunday from May to September and we must have seen millions of Gerties.

2. (camp gay, also Gertrude) a general term of address to a fellow homosexual man.

1918		R.H. Knyvett ‘Over There’ with the Australians 59: Another real artist played the mandolin, and when he appeared with it first of all he was greeted with cries of ‘Gertie!’ As he played, however he held the boys spellbound [...] though many still held that a mandolin was a ‘sissy’ instrument.
1933		J. Conroy Disinherited 192: Powdered, perfumed and rouged men strolled among the benches and occasionally accosted a bum [...] They addressed one another as ‘Agnes,’ ‘Gertrude,’ or some other feminine name.
1958		A. King Mine Enemy Grows Older (1959) 37: He turned out to be a Negro pansy [...] and his name was Gertrude.
1959		W. Burroughs Naked Lunch (1968) 173: Oh Gertie, it’s true. It’s all true. They’ve got a horrid gash instead of a thrilling thing.
1966		F. Elli Riot (1967) 71: There’s one from Gravel Gertie, too.
1990		(ref. to 1944) A. Bérubé Coming Out Under Fire 61: Soldier slang gave these men feminine nicknames such as [...] Gertrude.
2002		(ref. to 1926) in M. Houlbrook Sun among Cities 210: Gertie was 26 when he was arrested in Piccadilly in 1926. [...] Detective Pearse told the London Sessions that ‘since 1922 for similar offences in the West End and south London he had been ordered five terms each of 6 months’.

3. (S.Afr. gay) a heterosexual woman.

1972

B. Rodgers Queens’ Vernacular 81: a straight woman; any woman [...] Gerties (Cape Town gay sl).