Gin wins easily even without bankai.
He wasn't having any real difficulty with Ichigo, the same Ichigo who had roughly twice the reiatsu of a captain. Ichimaru is smarter, faster and dealier than Kenpachi, he shouldn't need his bankai.
But if it comes to it, one hit and it's over. If it worked on hogyoku Aizen then it would erode Kenpachi away in seconds.