http://stadt-bremerhaven.de/nexus5-film-gravity-nutzer/
Apparently gravity is free for people who have the Google Nexus 5. To add it to the library, open the play movies app, and it should be there, waiting for you. Otherwise try searching for it under "Mystery and Suspense" category and it might be there.
Can others here in Aus confirm/deny this working for them ?
Worked for me on my Nexus 5 thanks.