If you are not running Debian GNU/Linux then you can either download the source tarball and build it manually or checkout the latest sources using git and combine them with the appropriate gcc tarball and then finally build it manually.
Combined GNU Modula-2 and patched GCC tarballs exist at http://floppsie.comp.glam.ac.uk/download/c. Search for files which look like gm2+gcc-version-GCC.tar.gz. For example you should be able to download the latest version of GNU Modula-2 and GCC using these commands.
$ wget http://floppsie.comp.glam.ac.uk/download/c/gcc-8.2.0+gm2-git-latest.tar.gz $ tar zxf gcc-8.2.0+gm2-git-latest.tar.gz