Unsupported mean no instructions are available. You will have to install/test - figure it out how to build for yourself (all hypervisors)
If you followed all of the link that I posted you will see old documentation on Gentoo/Whonix. However they were abandoned.
https://whonix.org/wiki/Dev/Gentoo
https://whonix.org/wiki/HardenedGentooTG
Whonix 13 (current stable) build Docs:
https://whonix.org/wiki/Dev/Build_Documentation/13_full
Whonix 14 (soon to be released) build Docs: