I would guess that a significant number of Witcher 3 owners came to the game by way of The Witcher and The Witcher 2: Assassins of Kings. But there are bound to be quite a lot of newcomers too, and it's reasonable to assume that a goodly number of them will say to themselves, "I quite enjoy this Witchering business, and would like to know more about previous adventures I may have missed." And for those folks, GOG has a deal.
The retailer announced today that Witcher 3 owners who purchase the "expansion pass" before 11:59 pm PT on June 8 will be given free copies of The Witcher and The Witcher 2: Assassins of Kings on GOG. The pass includes two new "epic adventures," totaling an estimated 30 hours of play on top of the original game: Hearts of Stone, in which Geralt pursues a contract on behalf of the mysterious Man of Glass, and Blood and Wine, which leads him into the previously-unseen region of Toussaint, a land untainted by war but, inevitably, filled with dangers nonetheless.
Both expansions are expected "in the coming months," which, based on their original announcement, means October for Hearts of Stone, and early 2016 for Blood and Wine. The free games will be provided in the form of GOG codes, which can be redeemed or shared with a non-Witcher-owning friend until August 31.