"Open" is about accessibility to the development process and result products. It's not about conforming to the broadest possible "port" base.
Frankly, if there's something that Gnome needs (or just can get significant value from) that is available on Linux but not other platforms, I see nothing wrong with abandoning those other platforms until/unless they catch up. That's just progress.
That said, I don't see any evidence that this is actually happening. The systemd proposal was just that (a proposal), and while systemd is a pretty invasive change to linux boot and daemon management it really doesn't seem like something that couldn't be abstracted by FreeBSD in a way that would work with Gnome.
Frankly, if there's something that Gnome needs (or just can get significant value from) that is available on Linux but not other platforms, I see nothing wrong with abandoning those other platforms until/unless they catch up. That's just progress.
That said, I don't see any evidence that this is actually happening. The systemd proposal was just that (a proposal), and while systemd is a pretty invasive change to linux boot and daemon management it really doesn't seem like something that couldn't be abstracted by FreeBSD in a way that would work with Gnome.