dev-java/gcj-jdk is at version 4.7.3
gcc version: 4.8.1-r2

emerge gcj-jdk gives this output:
emerge: there are no ebuilds to satisfy "~sys-devel/gcc-4.7.3[gtk]".
(dependency required by "dev-java/gcj-jdk-4.7.3[X]" [ebuild])
(dependency required by "gcj-jdk" [argument])

since this machine is on unstable toolchain (gcc-4.8.1 glibc-2.18)
requirement by gcc-jdk for gcc-4.7.3 cannot be satisfied.

possible resolution:
gcj-jdk should be updated to version 4.8.1