I can't answer that because there are so many paths PG could take in a movie. That actually is one of her principle advantages over SG; she's much less well defined than SG, who would carry the weight of expectations from bother her own stuff like the SG TV series, and of relating to Superman. You could do nearly anything with PG, so long as you keep her a hero, much like Marvel had the ability to do anything they wanted with a character like Starlord for example in that first GoTG movie. You could go a light comic route for PG, like GoTG, you could take her on a Star Wars type adventure, maybe fighting Brainiac or someone like that. You could put her in an alien noir type movie. Maybe you skip directly to Earth, or Earth 2 depending on what you want to do, and the bulk of the movie takes place there. Maybe she's some sort of Kryptonian astronaut or explorer who got lost or trapped elsewhere when Krypton did the big sparkler thing. And again, SG herself isn't a topic that needs to be addressed in a PG movie; merely define PG clearly, and differently than SG, without ever mentioning her.
You would of course use whatever you decide her origin looks like, what job she does or wants to do, what her skill set is....all that and more to define how she responds and acts in every situation. Whichever, a key is to not Reyify her, and give her the flaws she's always had---I hate perfect characters who never struggle because they're so powerful, virtuous, smart, good at everything. None of that with Power Girl---a huge part of her appeal is her relatability, which in part stems from her flaws.