Cool thread. I have also wondered why Gulfstream has continued to market the rear galley even though it seemed to be made irrelevant with the G550 and G650.
G550 added a 7th row of windows to the original G-V design, making it irrelevant (from a window perspective) as to whether you put the galley in the front or back.
There is also the obvious weight balance question but I've never read anything to suggest that forward galley G550s have less range than the rear galley variants. While more weight at the front is generally considered detrimental, if the plane was designed from the outset to accommodate a forward galley there's no reason to assume that it would automatically perform better by moving the galley to the rear. Engineers can design the plane to perform any way they like.
From a passenger perspective, the forward galley is obviously strongly preferred. Keep the crew all in the front so that the passenger cabin has more privacy overall and the last seating section in particular, which is often walled or curtained off and serves as the owner's cabin, has direct private access to the rear lav and baggage.
Contrary to what another poster speculated, I can actually see the rear galley option as being possibly more attractive to charter or fractional operators specifically because there is less privacy. The rear galley gulfstreams always have completely open cabins while the front galley models often have cabin partitions. I think this is because the rear galley charter and fractional operators want an excuse to be walking through the cabin and observing the passengers. Less privacy = less opportunity for passengers to do anything too dirty.
But overall, I think that the reason that they still offer the rear galleys is because of nostalgia for the G-II, G-III, G-IV, G-V era, when they actually had valid performance reasons to place the cabin in the back. (1) moves the center of gravity rearward (2) removes the necessity to install windows in the weaker tapering section at the rear of the fuselage (3) putting the galley in the loudest part of the plane make the experience quieter for passengers.
If you ever browse controller.com you may notice that there seems to be a ongoing trend of ever greater proportions of Gulfstreams being fitted with the front galley. Almost no G550s have the rear galley and even with the G450 and G-V, the later models from more recent years have often taken the front galley even though it costs them a cabin window.