Gap beween groups
gap" refers to the difference in longevity or life expectancy between various groups, so there is no single longevity gap.
There is a gap between men and women, with women living longer, but that gap is narrowing. There are gaps between racial groups & socioeconomic groups. See http://www.Thenorthstarnews.Com/story/lost-decades-longevity-gap-widening-for-blacks-latinos-less-educated.