There is no app in the app store, but you can run the site as a web app. This will create an icon on your phone, and it will look and feel just like a downloaded app.
For an iPhone...
Go here, and click the icon shown. You can also do this from any page without using this link.
On Android phones, you should automatically see a prompt to add GTN to your home screen.
If you don't see it, or you declined the invitation, you may just have to wait until you see it again.