To quote The Wrap, here's their take on the ending:
"In Alfonso Cuaron's dazzling 3D survival tale, a burst of third-act diversions leaves open the possibility-to this viewer, at least- that Ryan never actually made it to the beach of some remote paradise.
Not in her Earthbound form, anyway.
There's just enough ambiguity to suggest that Ryan perished somewhere along the way, 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.
Or perhaps her final undoing ocmes when the astronaut who floats up to the airlock window (he's not recognizable at first, certainly not as Matt Kowalski) pops the hatch; Ryan is freaking out as he spins the crank, realizing that he's about to eject her into the void. A sleight-of-hand edit even suggests that's exactly what happens.
And here's where it gets squishy.
The events that take place afterward- Ryan's otherworldly encounter with Matt, her newfound resolve to carry on, her speech on the way down and that lucky-as-hell landing- play out like an absolution. If Ryan's soul has in fact detached and drifted early in the third act, it certainly spends the rest of the movie acting out the way things should have gone."
That is an incredibly interesting theory: and it's wrong. The problem with The Wrap (or The Wrap's source)'s theory is that Gravity is a story of rebirth. Sandra Bullock's character is being reborn in this film. She was lost, but by going through this life-changing experience, she has found life. So she certainly did not die in the airlock of the Russian Soyuz. Also, after her fateful scene with Kowalski, she seems to wake up, almost sealing the deal that Stone did not die in that scene and move on to a higher place of spirituality. I do however find some credibility in the fact that she might have died during the landing. Maybe she's landing in some spiritual heaven?
Still, despite there being a possibility of that happening, I still will say that Gravity is a simple tale of survival and a deep tale of rebirth. The alternate theories on the ending are possible, but not entirely plausible for the story that Cuaron was telling.