From https://wiki.gnupg.org/AgentForwarding
Configure local gnupg.conf
extra-socket /home/<user>/.gnupg/S.gpg-agent.extra
look for local socket path
gpgconf --list-dir agent-extra-socket
look for remote socket path
gpgconf --list-dir agent-socket
then forward the socket, SSH config
Host gpgtunnel
HostName server.domain
RemoteForward <socket_on_remote_box> <extra_socket_on_local_box>
=> should just work(tm). For smartcard access you need remote scdaemon as well.