I'm not a grower but I've tried the GSC from a variety of places. I've also had the Animal Cookies, Fortune Cookies, Thin Mint Cookies, Platinum GSC and Sherbert Cookies. What I've had has been all over the place in terms of quality but I have had some seriously good strains of this type.
I think it's OG Kush, Durban Poison, and maybe Cherry Pie.
The related types mix out the OG Kush for things like Fire OG (Animal Cookies). They claim there is some F1 plant in it? That is the mysterious one. Having tried both a high quality Durban and Cherry Pie as well and numerous high quality OG Kush and OG Kush crosses I think the mix I quoted is pretty accurate. This hybrid is balanced w/ the Durban and OG being strong polar opposites and the Cherry Pie also has Durban in it which makes me think it needs another indica to balance it out.
There is OG Kush in Chem Dawg so maybe that is what really ties this together on that solid 50/50 indica / sativa effect you get w/ GSC. Here is an interesting article about it's origins in High Times:
http://www.hightimes.com/read/inside-girl-scout-cookies