I use GPD to design my workflow,and use Eclipse web tool platform which depend on Eclipse GEF3.2 to write J2EE application.However,if I install GPD and GEF together,the GPD will don't work,that is,when I open processdefinition.xml or gpd.xml,it says that an error has occurred.After deleted the GEF.link in my Eclipse's links folder,the GPD will work well,and the Eclipse web tool platform will in trouble.
GPD is currently developed agains eclipse 3.1.x and wtp 1.0. So if you use a newer version there is a big chance it does not work. (Btw, this info is afaik in the docs and on the forum as well)