He can be selected, but Dowman was also a bigger guy. Yamal was similarly big as Dowman. JJ Gabriel is small even for his age group. It's just that he's so talented that despite the physical disadvantage, he's still impacting the game at a remarkable rate. If he were bigger/faster, his all around game would make a bigger fool out of the u18s last season.
You saw the difference against City in the FA Youth Cup Final. A couple of kids that played u19 and u21 reinforced the u18 City squad, and it made it much more difficult for JJ to impact the game. Because that happens in the FA Youth Cup as opposed to regular season u18 matches even though they're both u18.