Find out the owner of a DBus service name

You can get the caller PID via org.freedesktop.DBus.GetConnectionUnixProcessID:

gdbus call --session --dest org.freedesktop.DBus --object-path / --method \
org.freedesktop.DBus.GetConnectionUnixProcessID org.freedesktop.Notifications

prints

(uint32 737,)

so there's your PID. If you prefer dbus-send (as gdbus is GNOME specific) then:

dbus-send --session --print-reply --dest=org.freedesktop.DBus / \
org.freedesktop.DBus.GetConnectionUnixProcessID string:org.freedesktop.Notifications

The d-bus debug utility d-feet which is available as a package in many systems seems to be able to find the process id and command providing a service. For example, I ran it on a Fedora 23 xfce4 X11 systemd platform and selected Session Bus and entered the service name org.freedesktop.Notifications. It introspected the service, activating it, and showed the pid and /usr/lib64/xfce4/notifyd/xfce4-notifyd command: