| And I don't know how to help, really. Haven't really done messing with linux kernel config file directly. When you did the install procedures and compiled the kernel (I expect you did it yourself and not with genkernel) did you choose the compile your network card drivers into the kernel?
You could have done so by
cd /usr/src/linux
make menuconfig
and then choosing:
Device Drivers -->
Networking support -->
Ethernet (1000 Mbit) -->
Make sure Realtek 8169 gigabit ethernet support has and asterisk (*) before it.
If you want to try and recompile the kernel you can boot from live-cd and do few steps to get into your new installation. Just typing "chroot /mnt/gentoo" doesn't work however.
Before you 'chroot' you have to mount the new filesystem as was described here. After you have done that you can chroot to the new environment as was described here.
That should do it to get into your new installation while using the live-cd's kernel which has a working network and such.
After you can chroot to the new environment you can config your kernel to support your network card and recompile it.
Hope this made some sense and you can get it working. :) Just ask away if I was mumbling somewhere. | 
