Is the airspace at KGPT class E or G when the tower is closed?
To expand a little more on what @rbp said, the VFR Sectional Note is referring to the Class D and Class E (sfc) designations.  So, when referring the the Chart Supplement, which says AIRSPACE: CLASS D svc (1200-0500Z) other times CLASS G they are referring to the Class D and Class E (sfc) specifically—just as @rbp said. 

Now consider what the next level of controlled airspace is when the tower is closed and the airspace over KGPT becomes Class G. It would be 700’ AGL Class E airspace. The AIM goes on to detail this (emphasis mine):


  Where a Class D surface area is part-time, the airspace may revert to either a Class E surface area (see paragraph 3−2−6e1) or Class G airspace. When a part–time Class D surface area changes to Class G, the surface area becomes Class G airspace up to, but not including, the overlying controlled airspace.


Source: Aeronautical Information Manual (AIM), Change 1, May 26 2016, 3-2-5 Class D Airspace (13 MB PDF)

And goes on to explain in Note 2 of paragraph 3-2-5 (with added emphasis):


  Normally, the overlying controlled airspace is the Class E transition area airspace that begins at either 700 feet AGL (charted as magenta vignette) or 1200 feet AGL (charted as blue vignette). This may be determined by consulting the applicable VFR Sectional or Terminal Area Charts.