“But wait!” You say. “That’s not free if I have to already own something else!” Well, good news. The copy of Metro: Last Light Redux that you claimed from them in December counts. As does any Deep Silver game that you registered back when GOG Connect was a thing (and let’s face it, somehow everyone owns Saint’s Row the Third).
Just add Deadlight to your cart and when you go to check out, it will verify that you are eligible and adjust the price to free. If you already own it (say, from when they gave it away in 2017) you will get a code for a giftable copy.
Enjoy the game.