Hi Manuel,
I have a test.
First I create a GroupActivity in the source panel of GPD, then go back to designer panel, the graphical doesn't change. That is right, because the GroupActivity had not been supported. Then I modify position of some activity in designer panel, save, and change to source panel, the GroupActivity still here. So I think if we use GPD to write some codes by hand, it can maintain these codes for us, even if it have not supported this Activity or attribute. It is a good news for me.
I must say that I don't have much experisense on the GPD. One of the reasons is it is not finished yet. My job is developing a Web Designer for on jBPM 4, just like the jBPMEditor in the distribution. So we could use our web designer to design process definition directly.
I am not good at eclipse plugin development, so I could just wish the GPD could go on developing. At least, the commit of svn never stop.