Well…Gentoo never rose above the fray, in terms of people actually wanting to use it (and equally importantly pay for it). I did spend some time on the problem, and introduced some code for Gentoo way back years ago, but the lack of volunteers to help with it and my concerns about the package management and the incredible complexity of predicting what a Gentoo system really looks like (e.g. a Gentoo system can use several different init systems, syslogs, among other things…because these are fundamental to what the system looks like to the installer and to Webmin, it proved really difficult to make an installer that actually worked).
So, in short: No Gentoo support at this time. If someone were to volunteer to work on the installer, I’d provide what guidance I could and would be happy to setup an unofficial package repo for it. But, I don’t currently have anywhere near the time to tackle it myself; I’m swamped, as is, and if my previous attempts to support it are any indicator, it would be literally weeks of my time just to make it work. The install script is open source and in our github here: https://github.com/virtualmin/virtualmin-install
Some of the old Gentoo code is still in there, but it’s many years old, so it definitely needs an update, and was never complete, So, don’t expect to be able to make a couple of changes and run it…but if you’re really familiar with Gentoo (I am not) you may be able to get it working with a few days worth of work. You’d need to help me with the Virtualmin packages, as well, though I think there are already official portage packages for Webmin and Virtualmin (there used to be, anyway), which gets us halfway there.