The 3-4 we're running is one gap (I think; I forget the difference between one gap and two gap) and ask the down linemen to penatrate and be disruptive, not simply hold their blocks to let the linebackers roam free. If it was the latter, we'd need a big fat guy in the middle.
You're right. The Ryan brothers are one gap guys. The difference is defensive lineman responsibility. In one gap, the d-linemen (DE,NT,DE) are each responsible for "one-gap". This limited responsibility allows them to fire off and disrupt their gap. Two-gap defensive linemen are anchors, with their job being to hold their ground and read/react to the offensive linemen. The different responsibilities lend themselves to different players.
That is just my understanding, I could be wrong.