Green’s Dictionary of Slang

1. (US) a view of a fully exposed vulva.

1967		C. Cooper Jr Farm (1968) 185: She recrossed her legs, and I knew Cambridge must be getting some disturbing gappers.
1973		Oui June 13: Here is the little woman sitting on a bed throwing a full head-on gapper at me [HDAS].

2. (US) the mouth.

1981

Graziano & Corsel Somebody Down Here Likes Me, Too 96: When she opens her gapper you could see Brooklyn and hear her all the way back in Butte, Montana.