Goosebox is still a GN configuration...IF the manufacturer of the 5er states it's fine to use one, then have at it....I have a buddy with one and I find it easy to hook up to my 5er hitch than to his Goosebox, but it just may be what you get use to...You also need safety chains with the Goosebox. In most states you don't need safety chains for the Andersen Ultimate 5er hitch.
Again, instead of listening to all that's being said here, (including my post), I'd call the manufacturer of the 5er and see what THEY have to say about using the Reese goosebox with their 5er....I trust Lippert as far as I could throw my 5er....from them, I'd want it in writing. They're not much when building the frames for these 5er's, but will tell you to do something that they may not back in case something happens.
A phone call to the 5er manufacturer is such a silly simple thing to do than to try to glean from all these different answers one will get.....but common sense seems to be a rare commodity today....