What class is the airspace at KGCN when the tower is closed?
It may be an error on the chart, it should be either Class D or Class G, but it depends...

If the ATIS reverts to ASOS when the tower is closed, the airspace can be Class E. If ATIS/ASOS is not available, then the airport has to be Class-G. I would stick with the chart supplement here:

June 1 through September 30th, the tower is manned from 1300Z to 0300Z and is Class D. October 1 through May 31 the tower is manned from 1400Z to 0200Z and is Class D. All other times it is Class G meaning they do not have ASOS available. (AWOS is available by phone).

So according to the chart supplement, if the tower is not occupied the airspace is Class G.