Run lspci as root and see if the output includes anything to do with networking e.g on my system I get
Code:
00:0d.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+ (rev 10)
If so, that means that the card was detected fine and all you need to do is setup your networking using the instructions in the Gentoo Installation Guide.