Actually, GMOs need less pesticides than non GMO crop.
Roundup-resistant crops allow roundup-intensive farming [1] which does less* environmental harm than some more harmful pesticides (if you ignore the development of roundup-resistant weeds). But lower intensity, less productive organic techniques use fewer pesticides and do even less environmental harm.
[1] http://sitn.hms.harvard.edu/flash/2015/roundup-ready-crops/
Organic farming is restricted to naturally occurring pesticides which are often less effective than synthetic ones and so they need to be applied more often.
Also, organic yields are often lower than conventional ones, so more land may be needed to compensate. Clearing land is way more environmentally unfriendly than using pesticide, so any gains from organic techniques are nullified.
But i guess it depends on crop/climate and exact techniques
[0]http://www.science20.com/agricultural_realism/example_how_mu...