I've got the same problem. I havn't found a cure yet but it is definatly a bug with the GA-8KNXP motherboards not a driver issue.
The ethernet never worked on the first GA-8KNXP Rev 1 I got. I RMA'd it and it was replaced with a GA-8KNXP Rev 2 this worked fine until last week (3 months). Both times the device was showing in device manager but I couldn't get driver to install. The driver on the Gigabite driver CD should install fine, as it did when i first got the GA-8KNXP Rev 2. However it doesn't even display an install option.
I emailed gigabite support about this 5 days ago but they still haven't got back to me. I am currently having to use a PCI network card.
I have searched the web and this seems to be a recurring problem:
http://www.abxzone.com/forums/archive/index.php/t-74791.html
http://forums.extremeoverclocking.com/showthread.php?t=88971&highlight=ethernet