As Lord Giles said here:
-  Client sideClient side you need to connect with ssh -X(per session,~/.ssh/configto use it always)
-  Server sideServer side you need to have X11Forwarding yesin/etc/ssh/sshd_config(note thed). Also, some packages likexauthandlibxdmcp6, installed by default. Remember tosudo systemctl restart sshd.serviceto changes to take effect (use test mode before (sudo sshd -t), to check for any errors).
For security reasons, I wouldn't enable it to use it always, and in general only use it on very trusted servers.