Um, when you buy the disc version of the Witcher 3 you only buy one licence for the game.
The GOG key is for your own use because updating the game is handled only through GOG. Even if you consider the game stable enough so that you don't need any updates that doesn't magically create a second licence.
If you sell the key (which you are legally allowed to do) you have to give the buyer the discs together with the key and uninstall the game from you computer. If you don't do that the copy on your hard drive would be just as legal as a copy downloaded from The Pirate Bay.