If you have a network connection in the live cd environment, then either there's something missing from your kernel, and/or there's a problem with some configuration file
Does it still work when you chroot into your gentoo install? If so it's not a kernel problem. If however it works when you chroot, but then doesn't work when you restart not using the liveCD it is a kernel problem
But, I'm fairly sure that with dhcp your IP address would change everytime you connected (it is dynamic after all). With dhcp, you definitely don't need to specify an IP address anyway
To install gnome, you'll need to do emerge gnome, but the -k (--usepkg) option means you can do it from what's on your package CD, without downloading anything or waiting ages for compiles, as described here
http://www.gentoo.org/doc/en/handbook/h ... hap2_sect4