I have made it to get a nice gnome shell environment running under my new gentoo. It's all fine, except there are a whole bunch of programs built against gtk2 (Chromium, Pidgin, Thunderbird, Firefox), and they look strangely out of place. The ui of the gtk2 parts looks rather like windows 95 than gnome 3.
I have not yet found out how to configure the controls and icons. The instructions found on the gnome theme sites seemed to be ignored by my gentoo system or were made for Ubuntu.