The information in this site is the result of my researches in the Internet and of my experiences. This information below is solely used for my purpose and may not be suitable for others.
After installing x11-base/xorg-drivers and x11-base/xorg-server, X11 may exit with errors complaining, missing twm, xterm, xclock. When that happens, installing these components will fix the issue and hopefully X11 will start.
# emerge --ask twm xterm xclock