Log into GoG.com (same credentials as here), then ensure that you have registered your online code for Witcher 3 (should be in your retail package). Add the free dlc package to your account and "purchase", then go to your game shelf and download the patch 1.0>1.02 and 1.02>1.06 (or 1.02>1.07 if it has been released). You can also download the "goodies" if you want them, and the dlc (1-13)
Install the patches, then the dlc. Just run Witcher 3 using the GoG\The Witcher 3 Wild Hunt\bin\x64\witcher3.exe executable, rather than via Galaxy.