Thigh gaps are measured in male fingers, as in the number of fingers you could fit into the thigh gap. "Mind the gap" exclamations should be reserved for thigh-gaps of no fewer than four or five fingers.
Thigh gaps of this severity mostly happen to anorexics or girls whose thighs have been freakishly separated from being ploughed.
The phrase itself is derived from an announcement stated over the PA's on the London tubes (subway), which says in full "Mind the gap between the train and the platform."
-Holy shit! that chick is hot! and check it out "Mind the gap"!
-...yea...(takes mental picture)
"Dude, i can see your gap man...not cool"
"You need to MTG"