In my opinion, Microsoft Dynamics GP team might have hurried the release of GP 2015 RTM.
Think about this. You install GP 2015 and the user manuals that comes with that are GP 2013 R2 manuals. For instance, Visual Studio Tools manual that we get after installing the tool is of version GP2013 R2, as shown below:
Not just this manual, there are also other manuals that are basically for GP 2013 R2 but somehow got shipped with GP 2015 installation media.
Where can we get the latest manuals then? Answer is this link: Documentation and resources for Microsoft Dynamics GP 2015 and 2013.
First section on this post would be directing you to GP 2015 manuals.
QUICK UPDATE #1: I was wrong. Even this link seems to be not having *all* manuals. So probably the new ones are still under construction.
UPDATE #2: More relevant link, shared by Beat, is as follows: http://www.microsoft.com/en-us/download/details.aspx?id=45025
UPDATE #3: Microsoft would probably never update user manuals with each version of GP. This link apparently has got the word on this: http://blogs.msdn.com/b/developingfordynamicsgp/archive/2014/08/08/printable-manuals-for-microsoft-dynamics-gp.aspx