efl 1.8.3's configure has an --enable-gstreamer1 flag, which controls gstreamer:1.0 support. Thus not only gstreamer:0.10 is supported, as is currently the case in the ebuild.
it seems like this support was dropped in efl-1.8.4, which i plan to commit soon. With this in mind, i wont add gstreamer-1 support to efl-1.8.3 in the overlay. If you want gstreamer-1 support, please ask upstream to add that support again.