Gulfstream offers this option because customers request it.
My opinion is that the aft galley is a better choice. One reason not already mentioned is because of crew rest certification. Some (not all) FAA POI's require an aft galley for certification of the crew rest area, feeling that the noise and smell from the galley prevents adequate rest.
I believe all 550's have a forward lav, I've not seen one without, but there are two types of lavs, one where the crew rest should be, and another a "pocket lav" where you must open two doors.
For you Bombardier fan boys, I watched a 605 get grounded last month and require a new $30,000 ground spoiler control card due to the CA spilling coffee in the galley, and it dripped down into the avionics bay. Ooooops! I'll stick with my "legacy" 550 that dispatches always and can make it from Hong Kong to London without refueling!
FR