From what I saw on their website, GPhi started as a local w/ the intention to affiliate. So that could be a great way to start.
I would post fliers EVERYWHERE on campus. Especially the Res Halls. But remember, great sisters come from all walks of lives. So even the girls you think won't be interested at all could be your biggest assets!
Definitely get an advisor. Preferably a Greek one, male OR female. They'll have great advice and good conections.
I would email the girl you know in GPhi just to let her know what your up to. Just keep it to the point. An FYI. But, I wouldn't count on them for names. If they want to help; they'll come to you.
You'll do great!