Because we aren't certain about the effects of GMOs, we must consider one of the guiding principles in science, the precautionary principle. Under this principle, if a policy or action could harm human health or the environment, we must not proceed until we know for sure what the impact will be. And it is up to those proposing the action or policy to prove that it is not harmful.
Imagination is vital to precautionary judgement.
No precautions, and no precautionary principle, can avoid problems that we do not yet foresee. We need a stance of problem-fixing, not just problem-avoidance.