I have seen this around the net before and I vaguely having a similar issue ages ago and I do not remember how to solve it.

I am booting the amd64 version of the minimal install disk off of a usb key and I am able to use my usb keyboard mouse combo just fine in grub. The problem occurs right when gentoo starts I loose all keyboard functionality. I have also tried to boot the regular x86 install disk from an actual cd-rom and it does the same thing.

Some have mentioned that they have this same issue with usb keyboards when attached to UHCI controllers. I am not sure what this system has on board it is a ASUS m2n-32delux board. I have tried to boot it with and without legacy usb support in the bios as well. Only thing that happens is without legacy support I loose the keyboard in the grub menu. Note that I cannot use a converter on this keayboard as the keyboard and the mouse are wireless and connect with only one usb connection.

I ran into a similar problem installing Gentoo on my Macbook.
I resorted to booting with a Xubuntu CD.
... then just proceeded to install Gentoo.