All you have to do is sign up for GOG Galaxy beta. You can sign up here: http://www.gog.com/news/giveaway_aliens_vs_predator_classic_2000 Game have added support for Xbox 360 controller and has been updated for modern systems, just so you know. Promotion ends specefically at Friday, October 17, at 9:59AM GMT
It will be GOG take on Steam I guess. You can sign-up here as well: http://www.gog.com/galaxy Edit: "Truly gamer-friendly, DRM-free, online gaming platform", that's what official Galaxy page says.
You won't be forced to use it, it will be optional. The fact that they giving away free game for beta testing imo it's pretty cool (especially that it have been updated). @nhlkoho I think You will have access to Your whole library (just like in Steam), not sure how it's handled in the beta, though.
Great game, I still have the cd-rom version from 2000 which is what got me into pc gaming. Also have it on steam which I got for around 3 bucks during one of their sales
Go to http://www.gog.com/redeem and redeem the key that is in the email. You will need to create a account with GOG after you have redeemed it, that's if you haven't already got one though.