The area that forms between a girls legs and her ass when standing in an upright position.
"Check out the gappage on that chick" or "Oi fellas, you could park a mini in her gappage, have a look!"
by Scottyp September 17, 2003
Noun: angle of lifting of butt cheek to allow silent and comfortable escaping of gas from the bottom
Insufficient gappage will produce a deafening bottom bark.
by CathyV April 09, 2011