While the total ice-time says differently, Franson is essentially the Leafs' #2 defensemen, behind Phaneuf. Both players are the only two defensemen on the team who logs big minutes on both the powerplay and penalty kill.
As for Gallagher, he is rarely used on the PK, because of the depth of penalty killers they have on the team (that will hamper his ice-time), or else he'd be averaging upwards to 18 minutes a night. You also have to factor in that this is only his second year in the league. So, whose to say, that he's nothing more than a 2nd line tweener...? He's tied for 2nd in goals with 18, and 4th amongst forwards in points.