Our very dear
trentonlf has posted two great giveaways, but probably you can't enter the HITMAN giveaway (or see all the others on SteamGifts). If you're like me a got at least Episode 0 (the prologue) or Episode 1 when they were free, then that's the problem. Having at least 1 DLC from the game, SteamGifts marks you as having the full game, so you can't enter or see them.
https://www.steamgifts.com/go/comment/v4lmy6j https://www.steamgifts.com/go/comment/FnQnyHu https://www.steamgifts.com/go/comment/7der0pX There's only one solution for this problem, but you need to be brave and be sure to do it (:P), and that is to use the option to remove permanently the game from the account ("Detail View" —> Select "HITMAN" —> "Support" —> "I want to permanently remove this game..."), accept to remove it, and then sync your account with SteamGifts. After that, you should be able to see and enter trentonlf's and all the other giveaways.
EDIT: It seems you can recover removed games, this way:
https://www.youtube.com/watch?v=eV0ADs9a7Rs