Green’s Dictionary of Slang

a gynaecologist.

1920		Bulletin (Sydney) 15 Apr. 24/3: A learned obstetrician and gynaecologist who, when lecturing to students, wears returned soldiers’ uniform (though the Lord only knows what the Diggers wanted with a gyno. expert) has made some interesting experiments on this subject.
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 110: I’m a full-fledged gyno.