As far as eth0 goes, the handbook does talk about network interface
names other than eth0. See below for details.
http://www.gentoo.org/doc/en/handbook/handbook-x86.xml?style=printable&full=1#book_part1_chap3
Automatically Start Networking at Boot
To have your network interfaces activated at boot, you need to add them
to the default runlevel.
Code Listing 2.7: Adding net.eth0 to the default runlevel
# cd /etc/init.d
# ln -s net.lo net.eth0
# rc-update add net.eth0 default
If you have several network interfaces, you need to create the
appropriate net.* files just like you did with net.eth0.
If you later find out the assumption about the network interface name
(which we currently document as eth0) was wrong, then
1. update the /etc/conf.d/net file with the correct interface name
(like enp3s0 instead of eth0),
2. create new symbolic link (like /etc/init.d/net.enp3s0),
3. remove the old symbolic link (rm /etc/init.d/net.eth0),
4. add the new one to the default runlevel, and
5. remove the old one using rc-update del net.eth0 default.