Oh okay!!
I've met with your colony a couple of times already, welcome to GC!!
I've been in your shoes before as I've started a chapter at my school. So the biggest thing is BE SEEN BE SEEN BE SEEN!! And don't just be seen with your letters chillin...be seen doing what we do best, SERVE. Try having a campus-wide service project on your campus in which everybody can help. If you want to sponsor it with another organization you can, it doesn't necessarily have to be another GLO on campus. Help out with a move-in or another event with the Freshmen if Spelman permits you. I would really focus on the service projects--that's what we're about! People need to see that. But that doesn't mean that you can't host fun events too. Maybe plan a social event that everyone can attend. If your school has Meet The Greeks or something similar (I know we do here at GSU, it was the other day), participate in that.
Also another thing that gets looked over a lot is WEAR YOUR LETTERS! You'd be surprised at how much interest in GSS there is, and maybe that person didn't know that you were a member. When I was in undergrad I displayed something with letters EVERY DAY (I usually carried my books in my GSS bag). Or wear a nice pin or our colors if it coordinates with your 'fit.
Be ready to talk about Gamma Sig whenever you are asked! Be positive when you talk about the sorority and why you joined (if they ask you). If you all have an event or a service project coming up, extend an invitation You don't have come across as pressed about them joining, but one thing I got a lot in undergrad was that I was approachable and nice when it came to interests. Kindness gets you a long way!
Since ya'll are at Spelman you have something that a lot of colonies DON'T have--a lot of sorors in the area that will support you. Since I pledged close to 10 years ago *gasp*
my ideas might be a little outdated. I would definitely get in contact with the sorors at CAU and GSU since they are in the same city and they can share some ideas. But I sent you a PM anyway, to see if there was anything specific you needed.
Good Luck!!