All of gcc releases 4.9.0 4.9.1 4.9.2 4.9.3 do have one slot 4.9 If upstream has changed their version numbering such as gcc-5.2 is just a bugfix of gcc-5.1 then just one slot 5 should be sufficient?
i was waiting to see how versions shook out in practice. documentation is here: https://gcc.gnu.org/develop.html#num_scheme
i've updated the SLOT now for gcc-5+: http://gitweb.gentoo.org/repo/gentoo.git/commit/?id=9cfef505387b4bfce445217fd96fcd67a3f3b57d