I ran into this issue as well, as it turns out it's 'cause I already had Fallout from a previous giveaway. If you click the little 'x' beside the games you already have from the list shown when checking out, you can uncheck the box that says you're gifting the pack and proceed to check out as normal.

I seem to be getting 404s and 403s at random from the registration form's target URL -- perhaps they've gotten more interest in this offer than they expected.

And now I've just gotten an email purporting to describe my order, which I made through a freshly created account by virtue of never having used GOG.com before.

The email lists seven games, only four of which I've previously heard of, and none of which is a Fallout game.

What is going on over there?

Same for me - just went back to the fallout offer page (http://www.gog.com/promo/fallout_series_giveaway_winter_prom...) and redid the "checkout now" and it then added them to my just-opened account (I was logged in at the time).

You get a few free games upon account creation at GoG. This is unrelated to this Fallout special deal.