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...
Step 1
Go here, and click the icon shown. You can also do this from any page without using this link.
For Android...
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.