While it makes sense that gnome-packagekit should depend on packagekit if it can't function without packagekit installed, that then suggests that gnome should recommend gnome-packagekit rather than depending on it.
Having packagekit installed has (or had, it might not anymore) some system-wide side-effects, such as letting gnome-packagekit invoke the systemd offline-upgrade functionality, which isn't well-established or tested in Debian. While I'd love to see Debian using that infrastructure eventually, right now having packagekit installed may lead to serious unexpected problems. - Josh Triplett
