No boot/swap does not have to be on sda. You can boot up to your livecd that you used to install gentoo then mount your partitions
mount /dev/sdb3 /mnt/gentoo
mount /dev/sdb1 /mnt/gentoo/boot
make sure to use your partition scheme
mount -t proc none /mnt/gentoo/proc
mount -o bind /dev /mnt/gentoo/dev
chroot /mnt/gentoo /bin/bash
export PS1="(chroot) $PS1"
this will chroot you into your install and you should be able to get the grub command again
as far as 160 gig being a waste that all depends what you are going to be using it for. You can make your home dir 30 gig and leave the other 130 gig to be used for whatever and just mount it were and when you want.