Corina,
First off, good luck tomorrow!
If I were you, I'd start with a relatively small core group of women - such as the 18 women who want to join an NPC sorority but didn't get into Gamma Phi. I would not start postering all over campus until you know for sure that a sorority is interested in colonizing. Otherwise, you're likely to get a flood of women interested in joining the new sorority, and when you're at the stage of (1) picking the sorority that will come to campus and (2) getting approval from the administration to allow colonization, it's easier to deal with 18 women and their opinions than, say, 100.
18 is a good sized group - my local only had 10 members when we went national!
Once your core group and an NPC sorority have decided you're a good fit, *then* I would start postering and looking for additional new members. The NPC sorority might have you pledge and then hold rush for additional new members who would then be initiated with you.
Note that once your new chapter is established, your sorority and Gamma Phi will be working together and forming a campus Panhel. Just something to keep in mind - don't make enemies when you don't need to.
As for your Gamma Phi friend, in the membership selection process she only has one vote and probably not much say. She may just be upset that you weren't extended a bid, and she may not know how to talk to you. Just a thought.