Licenses libgcc, libstdc++, and gcc-runtime-library-exception-3.1 are additional permissions to the GPL. So having GPL-3 (or a set like @GPL-COMPATIBLE) in ACCEPT_LICENSE should be sufficient to accept gcc. I think the licenses should be joined by || rather than (implicit) &&: "|| ( GPL-3 libgcc libstdc++ ) LGPL-3" (This is for gcc-4.3*, 4.4* is similar of course.) Also, FDL-1.2 should be added for gcc's texinfo documentation.
I've changed it to the following (for gcc-4.3.4, but others are similar): LICENSE="GPL-3 LGPL-3 || ( GPL-3 libgcc libstdc++ ) FDL-1.2" because exceptions don't apply to all installed files. Of course, "GPL-3 || ( GPL-3 foo )" is logically equivalent to "GPL-3", but LICENSE is also informational.