Have you tried setting /startup/ssh-agent/type to the string "gpg-agent" in the xfce4-session channel in xfconf?
It's annoying that these configuration settings are so poorly documented. I've just told xfce4-session to just refrain from starting gpg-agent or ssh-agent (in my environment this is best left to systemd or to Xsession; xfce4-session tends to leave those processes behind on logout), but it took some digging to even discover that it was doing so. How about adding these settings, with their default values, to settings/xfce4-session.xml so that they are at least visible to users in the Xfconf editor? The interaction with /compat/LaunchGNOME will still need to be documented separately, though. Maybe it would be even more user-friendly if xfce4-session discovered the user's enable-ssh-support setting through gpgconf, but I'm not sure it's really worth the additional complexity.
