I've used
the G3 image once to install on a G3 Mac.
I've also heard of people having luck with
this PPC universal image.
Also, not that I like to discourage people posting here, because this is a great site, but forums.gentoo.org is a great place to get Gentoo specific advice/information/help.
There are instructions in the Gentoo Install Documentation that will tell you have to isntall Gentoo from another distribution on a running machine. You might want to try that.