This woman is Justine Wachsberger, and she plays a character in the movie por the name of Gianna, whom I'm not familiar with...can anyone tell me who this Gianna character is??

Gianna is a very minor character in New Moon.
She appear's in the book after Bella and Edward are reunited in Volterra and are being taken to see the Volturi leaders Aro, Caius and Marcus. She is the receptionist at the Volturi's headquaters, she is human and knows all about the Volturi and vampires.

Gianna is human who works for the Volturi and she hopes to become a vampire and registrarse them.

She is the human worker of the volturi.They keep her till she is no longer useful,then they eat her.Its not a nice job.You need to read the book,girl.