It seems that is should be in portage along with the rest of them. It got masked in package.mask for the gnome 2.6 mask, and works if I copy a different gst-plugins-* ebuild over and rename it.
not forgotten, but not reliable working, so i didn't put it in. And not all plugins have been added at this time, it is after all p.mask stuff.
open
got added with a note that it is known broken on some hardware