Besides the great price (a 2015 K1600GT is worth $18-20,000, I paid $15,000) part of the reason I bought the GT was the hope I would spend less farkling it than another GS. During my search I found a 2014 GS. The GS was loaded but since the owner was upgrading to a newer GS he stripped ALL of his farkles off the bike. Blank canvas, yes, but expensive! The GT was also a blank canvas but since it came with bags I figured (hoped) that I would spend less. The GT also had 2 1/2 years of warranty left, the 14 GS 6 months.
When I owned my 96 R1100RT I rode it for the first year with nothing but a tank bag and one bag liner. I could carry my clothing in the bag liner and the bike stuff (cover, lock, oil, tools, etc) along with my off the bike footwear in the other bag. The tank bag contained my extra gloves, face shield, wallet, and whatever else I needed. Latter I added a Marsee seat bag and a BMW top case. I found the more capacity you have the more crap you'll carry.
I'm gonna try my best to get by with less on the GT. Considering how much smaller the GT's tank bag is than either the RT's or any of the GS' tank bags I'm probably gonna have to buy another seat bag.