The issue was that there's no language level definition of module implementation and layout so every build system may need to handle every single module implementation differently. The standard body decided to publish a technical report on this issue to ensure some level of interoperability across module implementations.

(it already does but in a dev branch version, not very handy as it requires a full reconfiguration of the environment variables to switch between this and the normal GCC)

The status is here: https://gcc.gnu.org/wiki/cxx-modules