DHCP by default is set to timeout after like 3 minutes. This can be a real pain if you're booting up looking to connect via wireless and have to wait for eth0 for 3 mins at boot.
To change the timeout in Gentoo, edit /etc/conf.d/net.
The following shows setting it to 10s.
# nano /etc/conf.d/net ... //uncomment the following line dhcpcd_eth0="-t 10"