GMOs result in more herbicide use. A report from 2001-2010 found that herbicide use increased by 26% as weed resistance increased because of GMOs.
These herbicides are made of glyphosate which is a toxic chemical; this chemical gets into the water, harms soil quality, and increases the susceptibility of crops to diseases.
This is all extremely bad for the environment. Additionally, farmers are actually having to do more tillage (which erodes the soil) because the increase of herbicide-resistant crops is creating a rapid increase in superweeds that are resistant to herbicide.