This post is flotsam related to some dependencies. If you note any mistakes, please let me know, I'm collecting information.
Gimp doesn't depend on systemd as far as I can tell. Gimp runs on Windows. It does depend on libgtk, whose dependency list is available.
There are reports that Gimp depends on systemd, but that seems to be an artifact of a package dependency tree on one system, not that Gimp actually needs systemd.
LoginKit doesn't seem to be complete, like some files were missing from commit or something. The README is attractive, though. It seems to only be attempting to provide services for logind.
Systembsd provides hostenamed, localed, logind, and timedated.
Systemd-shim provides c-group services, some unit file services, power/sleep/reboot services, and ntpdate services.
Neither code base has been updated in four months, and the code in neither one looks particularly well organized.
I suspect there are other things systemd provides that Gnome relies on, but those aren't immediately apparent. I'm not sure where to look to figure that out.
Before logind, Gnome used ConsoleKit to provide login support. Fascinatingly, the commit to remove support for ConsoleKit was made by Florien Mullner.
A lot of this stuff depends directly on DBus. In my opinion that is a mistake; the communication mechanism should be separate from the API interface.
In the next journal entry I will investigate how deeply libsystemd is tied into the init system, or whether it is truly modular.