Interesting, I checked and the only upstream component of GNOME I could find that requires systemd and doesn't have a compile flag for it is gdm. This isn't required to start GNOME (you can use any other display manager) and FreeBSD/OpenBSD seem to be maintaining consolekit patches for it downstream. So I see why there is little incentive to fix this upstream. (On non-systemd Linux distros the focus has shifted to staying compatible with elogind, but that doesn't work on BSD)