Gilbert had simply grown up
He was determined to contract Raven to save Oz he even killed people, since the Nightrays are a crude dukedom.
Contracting Raven did not make him mature, he just grew up to be an adult its only when he's with Oz (or drunk) that he reverts back to being immature, clumsy or just plain Gil,
Plus contracting the powerful black winged chain 'Raven' takes alot of responsibility. o_O