This is a common misconception - that some GMO plant can suddenly take over the whole area or something. This doesn't work this way - no cultivated plant is better adapted to the local biosphere than the local wild plants (we call them "weeds"). This is precisely why farmers need to put a lot of effort into weeding to keep their plants cultivated. Leave a nicely cultivated orchard for a decade, and it will all turn back into wilderness.
Many people thing farming is just like gardening, just on larger scale. It's not - farmers plan soil rotation for decades. And as already explained, *any* crops currently in use is GMO and has been for thousands of years. The fact that Greenpeace lobbied calling GMO developed after 1980 "the GMO" is just a manipulation.
Regarding soil etc I can only recommend Talking Biotech podcast as they discuss it all the time http://www.talkingbiotechpodcast.com/