So my current understanding:
* Gnome use to depend on ConsoleKit.
* ConsoleKit is no longer maintained, and no one is interested in maintaining it.
* As a result, Gnome switched to using the implementation from systemd instead, as it has needed features and is actively being maintained.
* Some people say this means it needs systemd running as pid=1, same say it doesn't. Am still confused here.
* Some people say that the parts of systemd that Gnome uses should be split into a separate package, so, in theory, it should be possible to install just those parts without installing all of systemd. However the systemd object to doing this (I missed the reasons behind this).
* Gnome is said to work fine even on platforms that don't have systemd installed. Does this mean that systemd is optional?