Guza wrote Gia as Sonny's sister into the 2003 Fan February episode. It was like the story he wanted to do but didn't end up doing cause McTavish and JFP created Courtney, and so he incorporated it into that. Basically Taggert had known all along about Gia both their half sister, since Brooklyn, apparently. Sonny was like, you knew but came after me anyway?
There was a scene with Taggert and Gia's mom, Florence, approaching Sonny pointing out he was Deke's son. Sonny shook his head no, and she pointed out that he looked just like Adele and remembers seeing Sonny's picture on the mantel. There was another scene around this same time of Gia brought up to Carly that her (Gia) and Carly's husband (Sonny) knew some of the same people, and mentioned Deke being a mentor to her brother. Carly nixed that conversation in the bud for obvious reasons and said it wasn't a good memory. Remember Taggert and Sonny originally hated each other and ended up on two different sides of the law growing up because of their different expierences with Deke growing up.
Gia being caught between Taggert and Sonny would have also thematically connected her in a way with Nikolas, because she would've been caught between two sides of a family that hate each other just like Nikolas was with the Spencers (cause of Laura) and Cassadines. In the Fan February episode, it was even mentioned when Sonny brought up he must be going to hell to have Taggert as a step brother, which Luke responded he understood cause what happened to him was worse -- his wife spawned a Cassadine.