Christie says he wants to cooperate. Does he?
Christie chose to hire private counsel rather than use state lawyers. Here, he apparently learned a lesson from Kenneth Starr's investigation of the Clintons. Two circuit courts rejected a claim of privilege for communications between the Clintons and White House lawyers on the ground that the privilege belonged to the government which had no interest in stonewalling a federal prosecutor. By hiring private counsel, Christie can hope to control information Gibson discovers.
Of course, we haven't seen the agreement with Gibson. Who is its cient? The Christie administration? Christie himself? Can the executive branch of a state be a client? If so, it will enjoy all Upjohn v. US protections and communications with staff will be protected with Christie controlling disclosure. If Christie alone is the client, he won't be able to claim privilege for communications with his staff. The press should ask for the retention letter. A refusal to release it will tell us a lot.
In any event, the fact that Christie went to outside counsel where he can hope to control the privilege and the duty of confidentiality, coupled with the fact that Christie has not said that he waives privilege and confidentiality, tells me that this decision likely has a different goal. The goal is to learn at least as much as the US Attorney and legislature will learn and hopefully before they learn it. This appears to be a defensive maneuver, not a cooperative one as Christie claims.
No matter what Christie says about cooperation, the legislature and the US Attorney are his adversaries. Chrisite doesn't know what they will find when they go through emails and subpoena witnesses. Gibson will try to find out for him and do so first. Unless he waives privilege and confidentiality protection, and work product protection as well, Christie's claim of cooperation should not be taken seriously.