My friend, Walecha has successfully build GNOME 2.30 from source on his Slackware machine. The screenshots can be seen at his blog post. He posted a GNOME build order on another post (based on GNOME 2.26) and i think there has to be some changes to build 2.30 from 2.26. Yes, GNOME build are definitely BIGGER than KDE and probably that's one of the reason why GNOME is now left away from Slackware besides PAM problem.
Before you build your own GNOME 2.30, please know that you might need to upgrade other packages which are the requirements of GNOME 2.30 and it's not yet satisfied in Slackware-Current, so you will have to compile it from source.
FYI, the blog post about the build order is in Indonesian language, but don't worry about that. Just read the order of the packages and you should be fine.