I added it to my cart, but it says it's $2.99... anyone?

thanks

It was a 2 day offer that has expired.

I guess the promo is already over. It's available for 48 hours only.

From the page:
Be sure to come by and pick it up before 10.59 GMT on the 14th of December, though, to get it for free.

You're too late amigo.

I never really thought about it, but I wonder if some people have gift codes.

It wasn't giftable while 100% off.

You register here for the Unofficial GOG Advent Calendar, log in, sign up for the Empire Earth Gold giveaway and hope to get lucky (since a lot of GOG users already got the game now, I think your chances for winning aren't that bad). You need to do it within the next 23 hours though, after that there will be different games to win. :)