Change "rc" to "openrc" after "Now that the network interface has been successfully stopped and started, the next step is to have it started when Gentoo boots. Here is how to do this.:"
The handbook states to run the "rc" command after running: rc-update add net.eth0 default
However, the "rc" command is deprecated. (Source: I just ran the command and a warning popped up) According to said warning, the new way to call the command is with the "openrc" command