Once again, science loses out to religion. When an organization, be it a church or a secular group like Greenpeace, declares one of their beliefs to be above scientific questioning, it becomes a religious doctrine, a sacred scripture.
Whether GMOs should be patentable or whether farmers should be able to buy GMOs and replant the crops from seeds they collect, these are matters for political debate and Greenpeace can have at it. Whether GMOs are harmful to people or the environment, these are matters for scientific study, and Greenpeace can either submit their studies to peer reviewed journals or shut up. Otherwise they are no more than creationists or climate change deniers.