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. 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 always 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 well-- 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, was in effect when the question was originally asked, and 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, 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. It is sometimes used in cases where the Class D airspace reverts to Class-E-to-surface, and the airspace in the "extensions" also remains Class-E-to-surface. 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 we see sometimes the tag "See NOTAMs/Directory for Class D/E (sfc) eff hrs" instead of the tag "See NOTAMs/Directory for Class D eff hrs". An example of this is KPRC at Prescott, AZ.
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 notation "See NOTAMs/Directory for Class D/E (sfc) eff hrs", while the remaining 10 have the notation "See NOTAMs/Directory for Class D eff hrs". There appears to be no particular reason for the variation.
Now that we've learned more than we ever wanted to know about Class-E-to-surface "extensions", let's consider one more thing. As a pilot approaches an airport where the Class D tower has closed for the night and the Class D airspace has been replaced by E2 Class-E-to-surface airspace, is there any way that it could possibly matter to him whether or not the E4 Class-E-to-surface "extensions" were still in effect, or had been replaced by E2 Class-E-to-surface airspace with an identical footprint? Due to some ambiguous wording in the FARs and some related FAA memorandum and other documents, it could be argued that it would actually make a difference to the interpretation of certain FARs whether a pilot was in an E4 Class-E-to-surface "extension" or an E2 Class-E-to-surface "Surface Area", although the distinction has apparently never been enforced in actual practice in the particular context of E4 airspace that changes to E2 airspace when a tower closes. See this related ASE question, and other linked questions and answers, for more.
