I did some research. Not (completely) true. If it were, there wouldn’t have been
attempts at building a Hardened Gentoo based Whonix-Gateway:
https://www.whonix.org/wiki/HardenedGentooTG
Here the links to some interesting talk elsewhere about it (the most recent,
to my knowledge… so there’s little hope that HardenedGentooTG will be
revived):
https://marc.info/?l=gentoo-user&m=145563722929288&w=2
( esp. the links in this email of that short thread:
'[gentoo-user] Re: Tails security implemetation' - MARC )
And here the main Whonix developer sums it up why Gentoo can’t easily be used: