The primary IDE bus in a Blue and White G3 uses the CMD646 chip. This is where the hard drive lives. (The cd plugs into the second bus, whose driver is already in the kernel.) Without the CMD646 driver in the installer kernel, you cannot install gentoo on a Blue and White G3. It would be worth installing the aic7xxx driver also as that is apparently a common SCSI card for these macs, in the form of the 2930-mac. (My G3 has one in it, I don't know if it came with it or not.)
rc8 livecd from dev.gentoo.org/~pvdabeel/testing/ has this fixed .config available from ~pvdabeel/kernel/ kde/gnome livecd following