sabayon ask for /etc/X11/xdm/Xsession file in order to start the edition of a profile, but this file don't exist, even if xdm is installed
I must apologize, sabayon is not on gentoo (yet), I installed it using the tgz package several months ago and I was testing it right now.
Could you try the one in the overlay?
I ran it briefly, and it did run. That's all I did with it, tho.