The US does not appear define a specific threshold – best effort only, but I believe the EU allows 0.9% contamination in organic crops. The identity preserved program only allows 0.5%, so that's theoretically a better bet than organic, if you had some reason to care.
I think that is hard, since non-GMO can still contain GMO. Organic is a bit stricter, but I think also not 100% GMO free.
(Personally, I don't care about GMO, but organic)