The version string in /etc/gentoo-release has recently
Gentoo Base System version 1.12.5
This produces an rkhunter OS string of:
Gentoo Linux 1.12 (i386)
or the equivalent for your architecture, which is not
listed yet in os.dat, causing an "unsupported OS"
warning from rkhunter.
However, simply adding:
999:Gentoo Linux 1.12 (i386):/usr/bin/md5sum:/bin:
(copied from the Gentoo Linux 1.6 entry) to the end of
the file appears to make everything work.