The CLI_DEFAULTS_GENTOO_BASED_SYSTEMD_OS refers to /sbin/service which seems contradictory for a systemd operating system. Does Gentoo have an init-system agnostic service tool? In that case the 'systemd' part should be left out of the constant name.

Yes, I forgot - You are right. Gentoo has support for more init systems (https://wiki.gentoo.org/wiki/Comparison_of_init_systems) and my solution works only for openRC. Maybe we can rename the whole branch to something like gentoo_openrc, but that makes the detection code more complicated...
But back to Your question - I am not sure if gentoo has a universal service tool and I think it would be slightly complicated to support (and maintain) all init systems supported by gentoo. So I would suggest to add a big note into documentation (and maybe a comment in source code), that only openRC on gentoo is actually supported and people are welcome to provide definitions for other init systems...