Won Giveaway but I received Guest Pass

you can

yes you can

Yes we can!

Nope.Look above... ^^

The option to change the mark remains as there are circumstances that warrant it. The most specific example is when Steam removes the game from your library for whatever reason, such as fraudulent payment by the person who gave it away. In that event you change your mark and alert SteamGifts support. Guest passes I suppose you could do the same with, but its easier to just mark as not received since you didn't get the full game.