Who Was Taggert's Sister Gia On General Hospital?

Because his villainous grandmother, Helena Cassadine (Constance Towers), brainwashed his brother, Lucky Spencer (then Jacob Young), Nikolas pretended to leave Gia for Elizabeth to trick Helena. Lucky also had feelings for Liz, and Gia was surprised when he kissed her. Helena had removed Lucky's memories of loving Liz, so Gia did the honorable thing by helping Liz use the Ice Princess — then the world's largest uncut diamond — to restore Lucky's memories and deprogram him.

Nikolas had given Gia mixed signals by kissing her and then pushing her away again. Determined to get to the bottom of things, Gia eventually learned about his plan after overhearing Helena ordering him to kill Liz. Nik's father, Stavros Cassadine (Robert Kelker-Kelly) had been released from a cryogenic chamber and was working with Helena on a nefarious scheme. Finally, the villains were defeated and incarcerated. 

The couple got engaged, and when Gia drove after having some celebratory champagne, she accidentally hit Courtney Matthews (Alicia Leigh Willis). Nik covered it up, making it seem like Courtney was at fault. Helena had escaped from jail and used the accident to blackmail Gia into getting her some money. Nikolas let Helena escape to protect Gia, but ultimately once the truth got out, Courtney dropped the charges. Taggert tried breaking up Gia and Nikolas, but that plan failed, and they stayed together.