Normally in Gentoo you would want to create an init script via rc-update to start a service at boot. However if there are a few miscellaneous commands you would like to run you can add them to local.start.
The local.start init script is similar to rc.local in other distributions. It is the last init script to be run after all other services have been started at the end of the boot process.
Add your commands to local.start file located here.
You can also make commands run when the system is shutdown. These commands should be placed in local.stop located here.