It’s interesting to read Greenpeace and other environmental groups’ writings to see what they are objecting to.
It seems they are primarily concerned about two types of issue; one is why I’d call “safety”: gene transfer/escape into wild species, unintended ecological consequences, human safety of consuming GM plants. Tho other group is around “licensing”: essentially a deep concern about going down a path where plant seeds are licensed instead of bought, and all the “Monsanto is evil” concerns.
I think on the safety point there is some FUD about what kind of gene transfer is actually biologically possible, but you can make a more general rebuttal that selective breeding has the same risks, it’s just harder to get positive changes into your genotype. Maybe Greenpeace would argue that we’re already endangering the biosphere a bit with selective breeding, and going any faster would dramatically increase the danger.
On the licensing point, I’m very sympathetic, but I think that is an argument for IP law reform or public investment in GM, not an argument against GM itself. But of course if you believe the safety concerns then you don’t want public or private research to happen because you think it’s prima facie harmful.