Well I obviously didn't read every title on MD but I guess that subjects of this theme would be so miniscule (like 50-100 titles out of 50k) so it doesn't worth the time and effort.
Edit: Moreover "age gap" term is very vague. How exactly would you define it? Is 1 year difference enough to tag the title? 5 years? 10? Where would you draw a line? And why X-1 is not good enough?
SHOTA AGE GAP CROSSDRESSING INCEST YAOI IS WHERE IT'S AT
Well, what BzzBzz said. @Seyuu What would you consider deserving of an age gap tag? But specifically, age gap is a subset of romance manga but doesn't immediately convey that it is. Age gap could be the age gap between two protagonists in an action manga, etc etc