gentoo (openrc) hostname respects /etc/hostname
Currently, gentoo with openrc uses /etc/hostname to set the hostname. In case of its absence, it uses /etc/conf.d/hostname as a fallback. NetworkManager compiled with -Dhostname_persist=gentoo uses /etc/conf.d/hostname only.
The behavior has changed since https://github.com/OpenRC/openrc/commit/353bb9bc9a0ab3c6650d72d2ceb14c990762a2a0 Openrc-0.22.4 was stabilized in November 2016
NetworkManager(hostname_persist=gentoo) fails to set hostname if it is specified in /etc/hostname and there is no hostname specified in /etc/conf.d/hostname