GOG.com have confirmed that Fallout, Fallout 2, and Fallout Tactics are no longer available for sale on the site.
"Due to circumstances beyond our control, we needed to pull the three classic Fallout games, that is Fallout, Fallout 2, and Fallout Tactics, from sale and remove them from our DRM-free catalogue."
MCV cites speculation that this has to do with the recent passing of rights from Interplay to Zenimax Media. If you've bought the three games on GOG.com previously they are still available for download.