I am unsure which download is most useful.
PowerMac G5 is a "New World" Mac, and it is 64bit only. So you need this image:
If it's the Gentoo experience you are looking for, obviously you're on the right track. If you are just trying to get a Mac up and running on Linux, I strongly recommend http://debian.org. I'm not usually a Debian user, but I run Linux on a lot of Macs (my main laptop is an iBook G4) and I find that Debian is remarkably good at managing the quirks of Apple hardware. No bias; just what I have found from running Linux on discarded Apple hardware over the past several years.