The gift sign is a natural sign, looking like you are giving someone a gift. Make both hands into a fist with your index finger extended and curled. (This looks like the ASL sign for 'X'.) Starting with both hands close to your body, move them away from your body, in a gesture of giving.
You can start to teach baby about the idea of a gift by making small presents to baby and making the gift sign. And encourage baby to make the sign to you before they give you something. This also starts to introduce baby to the idea of sharing, giving, and receiving.
Teaching the sign for gift is a good first step toward getting baby used to the idea of sharing - the giving and receiving that is part of the holiday season. It is also a good way to prepare you child to say thank you, so when holiday time comes around, she can shine.