Try reporting to Gmail Support (Score 1)

Contact Google's Gmail support (through help) and report a hijacked/ compromised account. They will possibly only be able to tell you that you need to change your password, but explain your situation to them. (You might also be able to "reverse lookup" the IP through a Google search and see what ISP that IP range is registered to.) They might be able to further assist you with information of ISP, in which case, if you contact an attorney who has knowledge of such things, you can file a disclosure warrant to the ISP to release information of which subscriber has been issued that IP address for that time period. It's all (more than likely) going to have to be handled in court, it it could possibly cost you more than you've lost, considering that the person that burglarized you probably doesn't have a dime to their name, but you can at least feel better knowing that you have served justice to someone who has wronged you. Good luck!!