I don't think it is just a matter whether defenses which line up in the gap is sound vs. PASS or not (I know some 12 year old kids that can throw the H--- out of the ball).
Vs. RUN - I have always thought is unsound to have players attempting to "crawl" thru gaps, ESPECIALLY vs. teams with little or no offensive line splits. You are giving the opponent BLOCKING ANGLES that will eliminate your PURSUIT, & to many teams (Wing-T, SW, DW, & more effective style current day offenses such as "I's" & Onebacks) that use "Gap Schemes" - you are playing RIGHT INTO THEIR HANDS!
I've coached kids as young as 8, & I do not KNOW of a defense that doesn't need good PURSUIT to the ball (other than near the goal line - when you don't have to "defend it depth", & a Gap-8 may be needed).
There are 8 gaps the offense can attack, & if you line up in a GAP DEFENSE - the odds of them coming to YOUR gap are 1 in 8, THEREFORE, you need to be ready to pursue to the ball 7 out of 8 plays.
Not trying to "rain on anyone's parade" - this is just my opinion.