Some distributions come with package sets for different versions of GCC, which you can easily install in parallel; others do not. Specifically, in Devuan ASCII / Debian Stretch, there only seem to be GCC 6 packages.
What's the right way to install additional versions of GCC in parallel to v6? Is there a special repository like the Ubuntu-targeted PPAs (e.g.
ppa:jonathonf/gcc-7.1)? I'd rather avoid adding apt sources which need updating for newer versions of Debian (e.g. sid).