global ssh-agent on ubuntu 16.04 with gnome 3 and systemd

global_ssh_agent.sh

# create a user service for our ssh-agent
mkdir -p .config/systemd/user/
cat > .config/systemd/user/ssh-agent.service <<EOF
[Unit]
Description=SSH key agent

[Service]
Type=simple
Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
ExecStart=/usr/bin/ssh-agent -D -a $SSH_AUTH_SOCK

[Install]
WantedBy=default.target
EOF

systemctl --user daemon-reload && \
  systemctl --user enable ssh-agent && \
  systemctl --user start ssh-agent

# disable ssh component in gnome-keyring
cp  /etc/xdg/autostart/gnome-keyring-ssh.desktop .
echo "Hidden=true" | tee gnome-keyring-ssh.desktop

# relogin and enjoy!

# PS: my .zshrc contains this line
eval `keychain --eval --agents ssh`

If you wish to run an alternative SSH agent (e.g. ssh-agent or gpg-agent, you need to disable the ssh component of GNOME Keyring. To do so in an account-local way, copy /etc/xdg/autostart/gnome-keyring-ssh.desktop to ~/.config/autostart and then append the line Hidden=true to the copied file. Then log out.

From https://wiki.archlinux.org/index.php/GNOME/Keyring#Disable_keyring_daemon_components

@dyslexicdev: I incorporated your hint 👍 thx!