Yes, if the code is under GPL, it has to be shared openly (eventually, as anyone who purchases the code has the same GPL license in it that gives them the freedom to do so). This is commendable particularly to help the contributors back and community at large, particularly the poor farmer community.
I am now advising Chapingo Autonomous University that has been serving poor farming families here in Mexico that is agriculture based, and hopefully their center is setup soon to learn how to use iDempiere, and modify it for agri industry. Of course, GPLed and published.