Apple PowerPC machines don't use GPT to boot, but Apple Partition Map.

Independently of this, Mac OS X since Tiger indeed recognizes GPT and can work with it. Not too surprising, since that was the first version of OS X to run on Intel. But PowerPC machines can't boot from GPT.

So in this sense, MorphOS supports everything needed to work from the PowerPC machine, and GPT is not one of them. Anyway, there should be some basic support for GPT, at least to read disks using it, but it might not work in every case.