I suppose you're using debian gnu/linux, right? then you'd only have to do a apt-get install gnome, but you won't get an up to date version of gnome this way. I know there are debian packages available for gnome 2.2, but a quick search at apt-get.org and google didn't get me any ppc packages. Maybe you'll be happy with the older versions of gnome, I'm not sure, gnome 2.2. made everything a lot easier and more consistent. maybe it's available in unstable for ppc, I'm not sure.
btw if you need a graphical tool for package management and don't want to mess around with apt on the commandline do a apt-get install synaptic and you should get a really nice graphical apt frontend