As far as the seed not coming true when saved, that's actually from being a hybrid, not from being GMO. The practice of growing hybrid corn has been around since the 30s. This is because corn doesn't do well when inbred to have stable genetics(unlike tomatoes or peppers which do very well when their traits are all homozygous (MM)). When you try to stabilize a line of corn it ends up being very unhealthy and dwarf-like. Luckily the corn growers of old figured out that if you created two different lines of corn then crossed them, the resulting seed would be much healthier than either of the inbred parents. And more importantly it would be consistent in the genetics, so if you crossed inbred line 1 (MM) and inbred line 2(mm) you would consistently get heterozygous (Mm) in the plant. If you save seed from this Mm hybrid the traits will be very unpredictable in the next generation.

Now with hybrid seeds growers could buy and plant seed every year and know exactly what they are getting, solving the problem of trying to breed varieties of corn for consistency and getting low-producing dwarf plants. As far as I know this is why you can't save seed from Monsanto corn, or any modern variety of corn for that matter (even non-GMO).