I am by no means a GNOME expert (I never use it), but if you feel like getting your hands dirty the freebsd-gnome@ mailing list might be a good place to start. I suppose work on GNOME3 is probably already being done, so you might want to find out exactly where help is needed before wasting time and effort on "double work".
SRCDIR is a directory you have to specify for the script to temporarily store the ports. DESTDIR is ports directory you will be using to merge the ports into for example /usr/ports. My snapshot of the ports is currently broken however so I will post a notice to them.
I find FreeBSD has been a very great desktop for a[ ]long time now. I guess it all depends on how you use your desktop. FreeBSD 10 is working very well with my AMD GPU and I must say that is a big time improvement. I have a tiny problem with my sound as it doesn't play my external amps on the sound card chipset, but it did work once, however, the next reboot it went back to the way it is. But that still didn't stop me from using FreeBSD as a desktop.
You mention OS X. OS X may contain "some" BSD code in it, its still nothing like FreeBSD at all. OS X with Aqua is much like Windows ALOT big time. FreeBSD will be the desktop everyone is talking about in 2014. I see a bright future ahead for FreeBSD.