Too late now, but for future reference -- one way to avoid the whole "anyone can enter" problem is to select the base game only when creating the giveaway. In this case, "Grey Goo" (which links to an app ID) instead of "Grey Goo Definitive Edition" (which links to a sub ID). That way only people who do not own the base game can enter.

(In some cases you get less CV, so that's the downside. Although in this particular case the CV is the same).

Anyway, thanks for the giveaway!

Thank you for the game! :)