From the gentoo networking documentation, it seems you will have to add an entry into /etc/conf.d/net
for 'sit0'.
For DHCP:
config_sit0="dhcp"
For static IP (replace with correct values):
config_sit0="192.168.0.7/24" routes_sit0="default via 192.168.0.1" dns_servers_sit0="192.168.0.1 8.8.8.8"
Now to start the interface with the new configuration, from the terminal:
/etc/init.d/net.sit0 start
And to enable on startup:
rc-update add net.sit0 default