How can a SteamGift be Revoked?

People will pay with stolen credit cards or the like, funds get recalled, game gets invalidated.

Anything can be revoked. Either a gift or a key (even humble bundle and such). It usually involves stolen paypal accounts that, when found, get their payments revoked and their purchases too.

Basically you can't own something that was stolen or obtained through illegal means.
So if someone's credit card information was used to buy something that gift can be revoked. Otherwise people would be able to steal stuff and "gift" it to people with the original owner not being able to get it back.
Edit:
I think there are some countries who ignore stuff like that. So, if your car is stolen and someone bought it really cheap you have no way of getting it back. They don't care.