Hickory: No. It has to be
authenticated via Steam.
TPR: Fo4 on DVD comes with Steam version. They didn't write "It will be NOT available on GOG.com".
Hey, Bethesda guys! I will not buy Fallout 4 if it will be not available on GOG.com DRM-Free. I will not pirate it but I'll focus on other DRM-Free games. Your choice: money from me via GOG.com or no-money at all...
The last two AAA games I bought on Steam, Pillars of Eternity and Witcher 3, both shipped without any DRM of any kind--just as they do on Gog. Steam doesn't even have to be running to run either games. Thanks to Obsidian and CDPR for making that decision!
But as Fo4 is Steam only, my guess would be that Bethesda has slapped some old-fashioned Steam DRM on the title--otherwise, there'd be no sense in not selling the game through GOG.