If you were not paying much attention to the news over the past couple of weeks due to the holidays then you may have missed the fact that Fallout, Fallout 2 and Fallout Tactics were pulled from GOG.
A similar problem appeared on Steam with the games listed but not available for purchase due to the game rights changing hands from Interplay to Zenimax on 31 December 2013.
With everyone being on holiday it looks like this issue will be resolved in the coming days now people are back at work. Bethesda posted a Tweet this morning to confirm they are working on it.
We’re working to return classic @Fallout games (1, 2, Tactics) to Steam and will provide an update when they’re ready to go.
— Bethesda Softworks (@Bethblog) January 2, 2014
With GOG’s titles being DRM free, the warning they issued last week indicates that these games will not be returning to GOG so Steam will be the place to pick these up on the future.