Add https://raw.github.com/stefan-langenmaier/eclipse-overlay/master/repositories.xml
to overlays section in /etc/layman/layman.cfg
.
Or read the instructions on the Gentoo Wiki, then invoke the following:
layman -f -a eclipse-overlay
After performing those steps, the following should work (or any other package from this overlay):
sudo emerge -av dev-util/eclipse-sdk-bin