According to Wikipedia, Nautilus was originally written by Eazel inc. However, it seems like a major part of GNOME today. Nautilus's website is apparently live.gnome.org/nautilus. So it looks like GNOME really claimed Nautilus.
Could it be that Nautilus continued to be developed by GNOME, and no one else so that's why it's officially part of GNOME? Also are there any other environments that use Nautilus by default or is it just GNOME? And if there aren't any, is this for technical reasons, or otherwise?