debug1: Requesting X11 forwarding with authentication spoofing.
debug1: Requesting authentication agent forwarding.
debug1: Remote: No xauth program; cannot forward with spoofing.

both machines give this...

$ emerge -pv xauth

These are the packages that I would merge, in order:

Calculating dependencies ...done!
[ebuild   R   ] x11-apps/xauth-0.99.0  0 kB

$ which xauth
/usr/bin/xauth