Fable 1 and 2 are both incredibly important, helping to shape the action game umbrella and intentionally going against the grain to redefine the concept of role-playing video games. While the first is on PC in the form of a Steam release, it is unfair to new and old generations alike that Fable 2 and 3 are trapped on consoles that aren't manufactured, anymore. GOG releases would mean much for the preservation of video game history.
As an aside, I've noticed that many people are voting for Fable: TLC over the others. I'll point out that Fable: TLC is already DRM-free; while you must purchase it through Steam, you own it once you do and can run it without Steam even being installed. And you can link the executable to GOG Galaxy. If you're going to vote in this initiative, I encourage you to make sure that what you're voting for isn't already DRM-free and to prioritize games that haven't had the chance to come to PC in any form.