Hello there. Alright.

Hello RoCoKo.

Steamgifts didn't see that you own this game because it's deleted from the Steam store. If a game's information is not available in Steams API then there's no way to check for it.
There's other things to consider though: The sites rules require you to only enter giveaway for games that you don't own, yet. So you should have checked before entering. While I understand that hardly anyone does that and instead simply relies to the site's settings that are just meant to help with that (which has it's limitations as explained above) there are tools that make this a really easy task. ESGST as an extension or Steam Web Integration as a script.
The first doesn't let you enter titles you own and the second shows you a green checkmark besides every Steam Store link for which titles you already own. That way one can directly see if something is already owned or not.
I can only recommend using SWI. Not only to make things easier for oneself in order to follow the rules but also to help avoid unnecessary tickets and thus trouble for yourself and the giveaway creator.

You should hide giveaways for the game so this doesn't happen again.