You should not need them all. You would only need them if you're trying to compile a program against a kernel that was built with that version of gcc (NVIDIA drivers are a good example).
Have you tried to uninstall some of the older versions (start with the oldest and work your way up) to see what else is getting removed. First package that comes to mind would be the older kernel headers.
Since it's part of the build environment, they are probably getting installed with build-essential and kernel headers (both are part of default install, correct?), but will not be part of the apt autoremove since an older kernel and headers may need them. I think that's how it works.
A good place to research gcc would probably be the gentoo forum/wiki as a sources-based distro they should have some detailed explanations about it.
I am most likely going to build a new copy of the build environment and eliminate all but GCC 6 and see what happens
Actually, why not go the extra mile and eliminate the build environment from VSIDO; remove build-essential and kernel headers as well. That should decrease the size of the iso significantly. You don't build anything from source for the iso do you? Most users take advantage of the Debian repos, and nvidia users practically have to now (installing the nvidia driver from their site is now a major pain in the ass). Most Sid users know what to install to create a build environment, and all the packages are in the repos.