The question of how Gravity ends should be pretty simple (and if you haven't seen it, you should stop reading this now): Sandra Bullock's character survives her ordeal or she doesn't. But there are some who see a lot that's open to interpretation.
[Spoilers ahead, obviously.]
When I initially left the theater, I almost immediately considered the possibility that the final scene, while positive, also made a sort of comment on the futility of human life in the face of nature. Ryan Stone's escape from space takes her through a trial of fire, then water, only to emerge on a great unknown land—she had survived but it wasn't clear she had been saved. My take, however, is maybe just a pessimistic take on the events as they are shown. Others have floated possibilities that what we see isn't what happens. (Suprisingly—or perhaps unsurprisingly—these theories somewhat resemble the alternate readings of the Breaking Bad finale.) Josh Dickey of The Wrap explained that, to him, the film "leaves open the possibility — to this viewer, at least — that Ryan never actually made it to the beach of some remote paradise." Dickey speculates that Stone might have died at some point in the third act, "most likely in the airlock of the Russian Soyuz capsule, moments after she shut down the oxygen supply to hasten what seemed at the time like her inevitable demise." What follows is the dream-like hallucination involving George Clooney's character Matt Kowalski, Stone regaining her resolve, and, essentially, everything working out for the best. A user on Reddit has also posed this theory, writing: "I was kind of expecting to be blueballed when she turned off her oxygen. I thought that was the end. Actually what if it was? What if she cuts off her O2, and starts to hallucinate, and experiences euphoria and a feeling of drowing and burning sensations because she's running out of oxygen?"
For what it's worth, Alfonso Cuarón knows how his movie ends. He told New York's Dan P. Lee that for him "there was an ending and the ending was: She walks." He's reiterated that there's a "rebirth" metaphor at play here, telling Marlow Stern of The Daily Beast, who also reads both Bullock and Cuarón's backstories into the film that: "It’s the optimistic scenario, the Darwinian chart at the end. She comes from the primordial soup, crawling out into the mud, and then she’s on all fours, and then she’s standing up curved like an ape, until she goes completely erect." Talking with Lee, Cuarón is adamant that she survives—"I want to believe that if she survived what she survived … she’s equipped to deal with adversities,"—though he also noted that he didn't want to overstate what happens. In the Stern interview he leaves the door open for alternate interpretations: "I happen to subscribe to that view—rebirth," he said. "But that doesn’t mean it’s the only view…or the prevalent view."
It seems only natural that audiences wouldn't necessarily take Gravity at face value. Though people have praised how real Cuarón makes the scenario feel—if not necessarily through rigorous accuracy—space is still mysterious, and we're not sure we really can believe it.