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.