2022 Update: The "Long answer" portion of the answer has been updated to reflect some recent corrections on the sectional charts. Rather than re-write the answer to reflect a situation different from what existed when the question was asked, the updates have been in the form of superscripts that key to footnotes. A more concise explanation of the current (corrected) situation in regard to the tags on the sectional charts themselves may be found in this related ASE answer, beginning with the paragraph that starts "You should be able to tell the same information from the sectional chart itself."
Short answer
The chart supplement told you what you needed to know.
AIRSPACE: CLASS D svc 1 Jun thru 30 Sep 1300–0300Z, 1 Oct thru 31 May
1400–0200Z other times CLASS G.
means that the Class D becomes Class G airspace when the tower is closed. It never happens that Class-E-to-surface "extensions" remain in effect when the adjoined airspace reverts to Class G, so they revert to Class G as well.
You could not have known this for certain just by looking at the sectional chart.1 Read on to learn why.
Long answer
"See NOTAMs/Directory for Class D/E (sfc) eff hrs" is the standard chart notation used in a case where the Class D airspace, and its associated Class E-to-surface "extension", both revert to Class G airspace when the tower is closed.
For example, we can find an identical notation on the chart at KARD at Ardmore, OK.
It never happens that a Class-E-to-surface "extension" remains in effect when a Class D airspace reverts to Class G airspace.
However, if the Class D airspace reverts to Class-E-to-surface airspace, then the airspace in the Class-E-to-surface "extensions" invariably remains Class-E-to-surface airspace. In this case, the chart notation often but not always2 simply reads "See NOTAMs/Directory for Class D eff hrs". See for example KMFR at Medford, OR. Unfortunately, in some instances, the tag "See NOTAMs/Directory for Class D/E (sfc) eff hrs" is used in this situation as well3-- hence the need to check the Chart Supplement whenever this label appears. More on this below.
Another way to definitively find out what the situation is at KGCN, is to check the FAA's "Airspace Designations and Reporting Points" document, current edition (August 2019) Order JO 7400.11D.
The Class D airspace at Grand Canyon National Park airport is listed on page D-138, including the notice "This Class D airspace area is effective during the specific dates and times established in advance by a Notice to Airmen. The effective date and time will thereafter be continuously published in the Airport/Facility Directory."
If the Class D airspace reverted to Class-E-to-Surface rather than Class G when the tower closed, we would find a duplicate listing in the "E2" airspace section of the "Airspace Designations and Reporting Points" document. It would be on page E-128, between the listings for Douglass and Prescott. No such listing appears. Therefore the Class D airspace reverts to Class G airspace when the tower is closed.
It never is the case that a Class-E-to-Surface "extension" remains in effect when the adjoined airspace reverts to Class G. When the Class D airspace reverts to Class G, so does the Class-E-to-Surface "extension". We can confirm this by checking "E4" airspace section of the "Airspace Designations and Reporting Points" document. The listing appears on page E-205, and contains the note "This Class E airspace area is effective during the specific dates and times established in advance by a Notice to Airmen. The effective date and time will thereafter be continuously published in the Airport/Facility Directory."
The August 3, 2017 version of the document, JO 7400.11B, which was in effect when the question was originally asked, gives the same answers. The relevant page numbers are D-139, E-129, and E-213.
A word of caution-- one would think that chart notation "See NOTAMs/Directory for Class D/E (sfc) eff hrs" would be a reliable indication that the Class D (and therefore the attached Class-E-to-surface extension) all revert to Class G airspace when the tower is closed. If the airspace all becomes Class-E-to-surface airspace when the tower is closed, so none of the surface-level Class E airspace vanishes, shouldn't the chart notation always simply say "See NOTAMs/Directory for Class D eff hrs"-- as it does in the example at given above at KMFR at Medford, OR?
In fact, the tag "See NOTAMs/Directory for Class D/E (sfc) eff hrs" is not a reliable indication that the Class D airspace, and the associated Class-E-to-surface extension, revert to Class G when the tower closes.4 It is sometimes also used in cases where the Class D airspace reverts to Class-E-to-surface airspace, and the airspace in the "extensions" also remains Class-E-to-surface airspace.5 This is an unfortunate artifact of some FAA arcanery.
In more detail-- the FAA actually has two different ways of handling the situation where a Class D airspace, with Class-E-to-surface "extensions", reverts to all Class-E-to-surface airspace when the tower closes.
The more common way is for the E4 Class-E-to-surface "extensions" to be in effect 24/7 and for the Class D airspace to revert to E2 Class-E-to-surface airspace. This is when we always see the tag "See NOTAMs/Directory for Class D eff hrs", as at KMFR at Medford OR.
The less common way is for the E4 Class-E-to-surface "extensions" to close along with the Class D airspace, and for the "extensions" along with the Class D airspace to be replaced with a single E2 Class-E-to-surface airspace with the identical footprint. In this case the cartographic staff has apparently sometimes been confused by the tag in the "Airspace Designations and Reporting Points" document indicating that the E4 airspace is not in effect 24/7, failing to notice that it is fully replaced by E2 airspace which is still surface-level Class E airspace. As a result, in this case we see sometimes see the tag "See NOTAMs/Directory for Class D/E (sfc) eff hrs"6 instead of the tag "See NOTAMs/Directory for Class D eff hrs". An example of this is KPRC at Prescott, AZ.7
This related ASE answer gives a list of 21 airports that the FAA has handled in this manner. Of these 21, 11 (KGCK, KJEF, KNHK, KCXY, KUNV, KNTU, KMKG, KRST, KGFK, KILN, and KPRC) have the sectional chart notation "See NOTAMs/Directory for Class D/E (sfc) eff hrs"8, while the remaining 10 have the sectional chart notation "See NOTAMs/Directory for Class D eff hrs". There appears to be no particular reason for the variation.
Footnotes:
1,2,3,4,5,6,8: Update 2022: this situation has apparently now been fixed (except for two remaining "holdouts" which are KNHK and KNTU) and therefore it appears that in the particular case of airports where the Class D airspace has one or more surface-level Class E extensions, the occurrence of the phrase "See NOTAMs/Supplement for Class D/E (sfc) eff hrs" on the sectional chart therefore now can be taken as a reliable indication that when the tower closes, the surface-level controlled airspace reverts to Class G airspace. After this correction, the phrase "See NOTAMs/Supplement for Class D/E (sfc) eff hrs" is no longer used in any cases (again except for KNHK and KNTU) where when the tower closes, the airspace in the "extensions" remains surface-level Class E airspace (and the Class D airspace becomes surface-level Class E airspace)-- even in cases where the oddities of the "Airspace Descriptions and Reporting Points" document are such that the airspace in the "extensions" is designated to switch from E4 surface-level Class E airspace to E2 surface-level Class E airspace when the tower closes. For more, see this related ASE answer. It still remains the case that if the Class D airspace has no adjoining surface-level Class E "extensions", then the phrase "See NOTAMs/Supplement for Class D eff hrs" will always be used on the label on the sectional chart, regardless of whether the Class D airspace reverts to surface-level Class E airspace, or to Class G airspace, so in this case the Chart Supplement must be consulted to determine which is the case.
7: KPRC in Prescott AZ, KMKG in Muskegon MI, and KILN in Wilmington OH are no longer good examples of this case, because their surface-level Class E "extensions" have been entirely removed.
