I also used openJPA some time ago. I just neded to drop the jars into GF and modify the persistence provider property in my persistence.xml. It even was easily possible to switch between toplink and openJPA. From my experience I can say that the openJPA devs are very cooperative, if you have any trouble using openJPA.
Regards
Heiko
[Message sent by forum member 'hewagn00' (hewagn00)]
http://forums.java.net/jive/thread.jspa?messageID=230800