GRP info is missing from docs, I'll write some bits that have to be included in ppc docs. Also, there is some unclarity about which CFLAGS to use for each cpu/compiler combination. Was somebody (robbat2?) working on a CFLAGS tool? If not, a part about it in faq should be handy. I saw debian/ppc has a list that for each system lists if debian/ppc runs on it, which stuff is still experimental... I think something like that might be handy for gentoo/ppc.
GRP info is in docs.