Currently, you can watch GasLand via YouTube. You can also rent or buy the documentary via Vimeo or you can buy the DVD via Amazon. If you can buy the DVD or rent the documentary we highly recommend you do so as it's the best way to support the filmmakers and ensure more important documentaries like GasLand are made.
This documentary was one of the first to shed a light on the issues with fracking, an issue that has consequently been explored in many other movies (such as 'Promised Land' starring Matt Damon, see where to watch it online here if you want to learn more about fracking).