Black, quick correction. The Gecko is based off of a G3, I think in the 600 series.
I had a hunch the next step would be the "custom" clarification. Promise I'm not going to be a broken record.
I'll just say customizing a G5 for consoles would mean re-engineering it to a point where you couldn't really call it a G5 anymore.
While they're all lovingly called PPCs, their philosophies aren't in the same book. It's more realistic to start with the same core that Sony and MS used, and customize "up" from there (as opposed to "down" from a G5). Is it *possible* they'd strip a G5? Yeah, but not practical. IBM already has a discreet line of PPCs R&D'ed from the ground up specifically for console/set top box solutions.
Now if I'm off base, I'll gladly say I'm a retard and don't deserve to feed off of the jam between everybody's toes (I'll make it my signature). I'm in no competition for relevance like some of the anonymous sources. I'd love for it to be a G5, "custom" or otherwise, because it would wipe the floor with the competition's CPUs. But there are enough things that make the idea lean towards "snowball's chance in Hell."