Maybe Raven didn't like him at first, or just wanted to toy with him for 10 years XDD. I do believe it was implied that Gil had to literally fight for his position, to finally gain the contract, though for what purpose I do not know as Bernard knew he was the only one able to contract with it. I guess rivalry, foul play and internal politics had something to do with this. This would explain why Gil was only able to approach Raven for the 2nd time and contract with it, 10 years later. Remember he was aiming for a legal contract, to which he needs to gain a blood sealing mirror/carcere (not exactly common stuff), which further backs this up.