If you want a service to start automatically when Gentoo boots up, you need to tell Gentoo to start it. Gentoo looks at /etc/runlevels/boot to see what it needs to run, so all you need to do is add a symlink in here to your init.d script. In this example, I’ll use exim.
cd /etc/runlevels/boot ln -s /etc/init.d/exim exim
Next time Gentoo boots, it will see exim in the boot directory and run the script. This isn’t just limited to boot either – /etc/runlevels also has directories for shutdown and system initialisation.