Green’s Dictionary of Slang

1. a prostitute who performs fellatio.

1939		D. Maurer ‘Prostitutes and Criminal Argots’ in Lang. Und. (1981) 116/2: gobblegoo. A prostitute who prefers intercourse through the mouth.
1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.

2. fellatio.

1946

in G. Legman Limerick (1953) 281: (also gobbledegoo) He knew how to do / A neat gobbledegoo.