@gabi >is it possible to achieve the same level of freedom as Trisquel (theoretically 100%) by liberating Gentoo?
Yes, considering that the Gentoo stage3 contains only free software as it doesn't have Linux in it.

You just set ACCEPT_LICENSE="-* @FREE -Artistic", manually install Linux-libre (all the Linux ebuilds are for the proprietary version of Linux, although the rt-sources ebuild has a USE option to run the Linux-libre deblob script) and carefully watch out for other proprietary software with an incorrect LICENSE.

You still have to fight for your freedom, as Gentoo developers don't believe in the concept of someone using Gentoo without proprietary software.