According to this article on Android Authority, it seems like Google is giving out digital copies of Gravity on the Google Play app for some lucky users. Not sure how this is being decided, but after reading about it, I checked the app on my phone and there it was, free to add to my library, which I immediately did. I'm not sure it's my kind of movie, but I know a couple people that had wanted to see it, so at the very least I can let them watch it.
Update: it's not just select users, it's also device specific. I got Gravity on my Nexus 5, but it doesn't show up in the My Movies list when I check my account on my laptop, and it also didn't show up in the app on my Nexus 7. The Shaun the Sheep : Big Chase movie seems to work similarly, since it showed up in the app on my Nexus 7, but not on my Nexus 5. So if you have multiple devices, check them all.
Update 2: according to Android Police, it's only on the Nexus 5.