How can I forward a gpg key via ssh-agent?

OpenSSH's new Unix Domain Socket Forwarding can do this directly starting with version 6.7.

You should be able to something like:

ssh -R /home/bminton/.gnupg/S.gpg-agent:/home/bminton/.gnupg/S-gpg-agent -o "StreamLocalBindUnlink=yes" -l bminton 192.168.1.9

EDIT: This answer is obsolete now that proper support has been implemented in OpenSSH, see Brian Minton's answer.

SSH is only capable of forwarding tcp connections within the tunnel.

You can, however, use a program like socat to relay the unix socket over TCP, with something like that (you will need socat both on the client and the server hosts):

# Get the path of gpg-agent socket:
GPG_SOCK=$(echo "$GPG_AGENT_INFO" | cut -d: -f1)

# Forward some local tcp socket to the agent
(while true; do
    socat TCP-LISTEN:12345,bind=127.0.0.1 UNIX-CONNECT:$GPG_SOCK;
done) &

# Connect to the remote host via ssh, forwarding the TCP port
ssh -R12345:localhost:12345 host.example.com

# (On the remote host)
(while true; do
    socat UNIX-LISTEN:$HOME/.gnupg/S.gpg-agent,unlink-close,unlink-early TCP4:localhost:12345;
done) &

Test if it works out with gpg-connect-agent. Make sure that GPG_AGENT_INFO is undefined on the remote host, so that it falls back to the $HOME/.gnupg/S.gpg-agent socket.

Now hopefully all you need is a way to run all this automatically!

In new versions of GnuPG or Linux distributions the paths of the sockets can change. These can be found out via

$ gpgconf --list-dirs agent-extra-socket

and

$ gpgconf --list-dirs agent-socket

Then add these paths to your SSH configuration:

Host remote
  RemoteForward <remote socket> <local socket>

Quick solution for copying the public keys:

scp .gnupg/pubring.kbx remote:~/.gnupg/

On the remote machine, activate GPG agent:

echo use-agent >> ~/.gnupg/gpg.conf

On the remote machine, also modify the SSH server configuration and add this parameter (/etc/ssh/sshd_config):

StreamLocalBindUnlink yes

Restart SSH server, reconnect to the remote machine - then it should work.