Problem with a giveaway that i cant enter

An unfortunate issue with the way Steam sends its data for other sites (like SteamGifts) to use.
Not much you can do except re-sync your account as soon as the free weekend is over, and hope for the best. (Sometimes, I hear it still takes a week or two to clear out.)

[edit] Also, in the future, best not to manually sync during a free weekend where you don't already own the games. Even then though, SG auto-syncs once a week. [/edit]

Someone once said it works if you put it on your steam wishlist and re-sync your account, but I've never tried that and don't know mif it actually works.