In mid-December GOG.com gave away Fallout games for free to kick off its massive Winter sale. If you wondered why that was happening, the answer is simple enough: both Interplay and GOG.com knew that they would soon lose the rights to sell these classic games as of January 1, 2014. So now if you want to buy any of those earlier Fallout games, you'll have to find them somewhere else because they are no longer on GOG.com. Of course if you purchased the games before Dec. 31, 2013, you can still download and play them (after all, you own them).
The disappearance from the GOG.com catalog coincides with a settlement between Interplay (who used to own the rights to "Fallout" but sold them) Bethesda (who bought them) and then got into a legal battle over them later on down the road. The settlement which awarded Bethesda the full rights to the Fallout franchise happened earlier in 2013 after many years of litigation over Interplay's plans to make a Fallout MMO. Bethesda argued that Interplay didn't have the rights to make the MMO and that its product would cause brand confusion amongst consumers.
The classic Fallout games will no doubt be available at other places in the future, but for now they are not available on GOG.com or Steam...