Not blue jeans. That last forever! "longevity gap" means how different groups live shorter/longer than others. For example, in some countries women live longer than men. Great question.
Gap beween groups. "longevity 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.