Try Archlinux PPC. You start with a very blank Linux and can add in what you need as you go along using their package manager, Pacman. Their Wiki has great guides to get you going if you are new to Linux in general. I also recommend you use the Net Install over Core Install, because then you get the newest packages. Their base packages are about 300mb (120 download). I'm not sure if this applies to PPC version, it goes for x86 and x64 though.
Also for a GUI, you are referring to a Desktop Environment. Xfce is somewhat similar to gnome, and is only about 15mb. It uses very little resource wise. If you wanted even less of a Desktop Environment on resources, try out Fluxbox