Well, let's try to sort this out.
* Which ebuild did you use? Mine, or your own?
* Did you update all the dependencies? (gnome15-core, gnome15-plugins, ...)
* What exactly are you trying to start? The desktop service? Or the config utility? or...?
* Could you post some output? Maybe I can be of help
As of my system, I am using KDE 4.8.3, and LXDE. I have GTK2 and 3 installed, and I am trying to use gtk3 over gtk2 wherever possible.
Are you using Python 2 or 3? That could a another problem on it's own.
Did you try running python-updater already? It fixes a whole lot of problems.