On Gentoo, Xsession is located at /etc/X11/Sessions.

--- a/opt/google/chrome-remote-desktop/Xsession
+++ b/opt/google/chrome-remote-desktop/Xsession
@@ -9,4 +9,4 @@
 
 # On Debian-based systems, just delegate to the distro-supplied Xsession script
 # to launch the selected session.
-exec /etc/X11/Xsession "$@"
+exec /etc/X11/Sessions/Xsession "$@"