Steven Gilmore was very likely the Lions # 7 CB in the team's pecking order. This year, that wasn't enough for Gilmore to survive the cutdown to 53. His release isn't surprising.
I think there's at most a 40 % chance Gilmore is claimed off waiver. While I hope the Lions and Gilmore agree on a PS contract if Gilmore clears waivers, I'm far from certain both parties will want to continue their football relationship. Especially, Gilmore.