Isn't it so that steamgifts does auto sync your account every 2 week or so to verify the games you own? It does that for me, otherwise I can't participate in any giveaway...
Yes, I think steamgifts autosyncs, I'm not 100% sure, but if your profile is private when it syncs, it won't update it (im not even sure if it zeros out your list). Private accounts are allowed, because "FRIENDS" can be allowed to view accounts, while keeping it private to the public.
So the giver/winner has to become friends to check.
If the winner refused to show you their games at all (either via becomeing friends, or it being public), then you can ask steamgift mods for a re-roll.