Gagne and Foulke are two completely different pitchers who really can't be compared at all.
Yes, Foulke was a good at a point in his career, but never as dominant as Gagne. Foulke relied on a 92 MPH fastball and a 78 MPH changeup in his prime. After getting injured he was throwing 86 with his fastball and guys were teeing off on him.
Gagne throws more than just 2 pitches so if his fastball does fall a little, he still has a nasty curve and great change. Noone knows if Gagne can bounce back, but if he does he'd be a big steal this offseason