GOG don't want to set the world on fire, they just want to impart Fallout in your (shopping) cart. The easiest way to do that, of course, is to make Fallout 1, 2 and Tactics completely free for the next couple of days, which is what they've done. Why? Firstly, because it's an apocalyptically good way to launch their Winter Sale (opens in new tab) . Secondly, because rights to the series are currently pending a change in ownership, which may see the classic RPGs removed from sale on the service.
"Sadly, to the best of our knowledge, we'll be forced to remove Fallout, Fallout 2, and Fallout Tactics from our offer," GOG write. "That doesn't mean, however, that you won't be able to download them if they are already a part of collection by then."
Basically, get over to GOG, "buy" the three games, and you'll be able to access them from your library even after they're removed from sale. Sure, you will then own Fallout: Tactics, but that's a small sacrifice to get a free copy of the two good early Fallout games.
That deal finishes December 14, 1:59pm GMT. The Winter Sale will continue on past that, though. Currently, it's also offering (opens in new tab) up to 80% off a selection of games released this year, as well as a choice between three mystery deal packages.