Different GMO's work different ways, but one type of GMO seems to be having a very positive effect on the environment. Unlike engineered crops such as Roundup Ready varieties, which encourage extra application of the herbicide glyphosate, the Bt variety has pesticides built into the crop and thereby reduces application of chemicals. A recent study found that use of the variety in India greatly reduced the environmental impact of agriculture there.
I'm still not sure where the knee-jerk anti-GMO sentiment comes from unless it's the same anti-science bent that makes people "not believe in" other scientifically established truths like climate change. Surely we need to be careful with any new technology, but mindlessly fearing it without knowing what it does can't be good!