One benefit I see with Gray is that he does know the players in the mix. He also knows more defense than the so called 3-4 we run. He has a wide variety of experience various schemes. He's also in-house, so not an unknown commodity.
I don't believe Gray has done a bad job as DB coach. I remember he had some good years as defensive coordinator in Buffalo. That is, until his last season there. I don't believe their downfall was really on his shoulders.
The more I think about it, and start shedding the aura of Pettine being on everyone coaching on that side of the ball, I'm thinking he just might be the guy they want. The Packers can find another solid defensive back from the past that could step in, and offer the same kind of guidance Gray offers.
I just convinced myself. Give the job to Gray, and let's see what he can bring to the defense. He did have a solid NFL career, and was a 4 time Pro Bowler. He's not dumb. He knows what it takes to succeed.