I'm trying to do X11 forwarding from my Galileo (gen2) to my Ubuntu x64 computer. I've
enabled X11 forwarding on my Galileo by editing /etc/ssh/sshd_config:
I still get
$ ssh -X firstname.lastname@example.org
X11 forwarding request failed on channel 0
Was editing /etc/ssh/sshd_config the only change you made to enable X11? When you reboot the board do you see the changes you made saved? When enabling X11 forwarding you need to restart the SSH server with /etc/init.d/sshd restart. Make sure the lines you edit are uncommented.
What openssh version do you have? Are you using the uclibc image, eglibc image or a custom image?