Not sure if it is a problem with my setup or not but I can not see gcc-7.3.1 in repos.

>emerge -s sys-devel/gcc

[ Results for search key : sys-devel/gcc ]
Searching...

• sys-devel/gcc
  Latest version installed: 6.4.0
  Homepage:
  Description: The GNU Compiler Collection
  License: GPL-3+ LGPL-3+ || ( GPL-3+ libgcc libstdc++ gcc-runtime-library-exception-3.1 ) FDL-1.3+

Also when running> revdep-rebuild --library 'libstdc++.so.6' – --exclude sys-devel/gcc

 !!! Dependant orphaned files: No installed package was found for the following:
        * Warning: "sys-devel/gcc-6.4.0" ebuild not found..
        !! Could not find ebuild for sys-devel/gcc:6.4.0
        !! Could not find ebuild for sys-devel/gcc
        Installed package: sys-devel/gcc is no longer available