Green’s Dictionary of Slang

1. (US) any small thing for which one has temporarily forgotten the correct name.

1942

in Leatherneck Nov. 74: Gizmo ... ‘giz’ [HDAS].

2. (US) ammunition, explosives.

1957

(con. 1943) A. Myrer Big War 188: Tagged by a torpedo we’ll get blown a mile high in the air with all this high-powered giz under our butts.

3. (US) the vagina.

1975		J. Wambaugh Choirboys (1976) 49: Guy tried to shove a Pepsi bottle in his wife’s giz after he caught her stepping out on him.
1981		J. Wambaugh Glitter Dome (1982) 98: From her sagging gullet right down to her chilly old giz.