I have the 2015 GMC 2500HD and I had the dealer install the gooseball turnover hitch (the GMC catalog version is a Curt hitch). It worked great. I added a GMC rubber truck mat ($65 as I recall) and cut holes in the mat for the ball and safety chain connects to peek thru.
To that I added the Andersen Ultimate Aluminum Fifthwheel hitch. 2 very good reasons for the Andersen. First, it's light 32# so it saves on the precious available payload, Second, it's easy to remove when not towing. Bonus, it's adjustable enough to handle that tall truck bed on the 2015 GMC/Chevy std. beds.
If you haven't decided yet, check out the youtube videos. Start with this video.
Amazing Ultimate Aluminum Fifth Wheel