Quoting Zac's summary:
"That seems like a reasonable solution. So, an ebuild can do
something like LICENSE="@GPL-2+" and that will expand to whatever
the definition of the GPL-2+ license group happens to be. When a new
version of GPL license comes out, we simple add it to that group,
and none of the corresponding ebuilds have to be updated."
See URL above for related message and thread.
I believe that this is obsolete, per the 20120911 council decision.