Try leaving it blank unless state income tax was withheld.  The W2G instructions state:

State Tax Information
Boxes 13, 14, and 15 and Copies 1 and 2 are provided for your convenience only and don't have to be completed for the IRS. If you withheld state income tax on a payment of gambling winnings, you may enter it in box 15 of Form W-2G. If you do, also complete boxes 13 and 14. A state identification number (box 13) is assigned by each individual state.