She is portrayed by Skyler Samuels.
Her first appearance was in the pilot. Their rivalry is dated back to kindergarten, when Gigi spilled juice on Alex's mat and then told everyone Alex had an accident. She also targets Harper in the episode Alex's Choice, and invites Harper to her annual tea party to crown her the Biggest Loser, showing she doesn't see Harper as a challenge.
Gigi does not torment Alex and Harper alone; she has a duo of girls that bow down to Gigi's many whims. Gigi hangs out with two girls who Alex and Harper refer to as the "Wannabes." The girls dress similarly to Gigi and underwent plastic surgery on their noses to look more like her. They will do anything she wants.
When Gigi's not there, Alex and Harper tell them that Gigi wants them to shave their heads, which they do to Gigi's horror. They also put there shoes on their hands and put on yarn wigs. The wannabe weren't smart enough to know Gigi hates yarn and almost found out that Alex and Harper were getting revenge on them.
GiGi's quest to make Alex look bad almost outs the Russos as wizards when she gets into Alex's private journal and lives in Alex's medieval fantasies. It is then that she discovers Alex's crush on Dean Moriarty. Other than Harper and Zeke Beakerman, she can be considered the character closest to finding out the Russo secret.
She doesn't appear in season three, possibly having been expelled or having moved away. However, "the Wannabes" aren't shown either, to see if they've lost their direction.
It is also implied that GiGi is wealthy, as mentioned by one of the girls in Alex's Choice, where she said she "would be prettier if she "could afford the same dermatologist as GiGi." Typical episodes involving GiGi, depict her trying to humiliate Alex and/or Harper, and receiving poetic justice, usually from Alex's end.