If a bunch of GM seeds land in your field, and they manage to survive (without some special fertilizer). The only effect they are going to have is boosting crop levels in the first year, and then due to their inability to reproduce they will just die out and have no effect on the next year. No long term contamination there. Or am I missing something?
Some crops are able to reproduce and some are not. Corn for instance hasn't been able to reproduce well for 50 years. That is not to say if you plant a kernel of harvested corn it won't grow. It just may not produce a very good crop. That is why it is called seed corn and not corn seeds. It's a hybrid. Has nothing to do with modifying plant DNA. It has to do with selective breeding. Soybeans that are Roundup ready will reproduce over and over and over just as soybeans always have been. Going back 50 years to open pollenated corn will work like soybeans. The harvested kernel will reproduce over and over and over with the same results year after year but at a seriously reduced crop yield. Going from open pollenated corn to the hybrid was a huge step in production.
Given typical farming practices, crops are rotated every year, so anything left in the field due to harvest loss or some (ridiculous) spilled seed, the next year whatever is left is considered a weed since a different crop has been rotated in.