Gravity isn't science fiction
Of course it is.
We actually do send people into space, and that kind of disaster could sort of happen.
But we didn't send anyone named Dr. Ryan Stone on space shuttle mission STS-157, and none of the other events in the film ever happened... so its CLEARLY fiction.
And it is science fiction because many of the antagonists/obstacles are consequences of the known rules of physics.
It handily meets any definition of science fiction I would ever care to use.
And that's really cool--what seems so much like SF is actually a real-life job that some people do everyday.
We all live moments away from science fiction. A fictional story about the challenge of escaping a car after it goes over a bridge into a river can be science fiction if the accident is modelled according to our understanding of science instead of just done for dramatic effect. The juxtaposition of the vehicles crumple zones with how they'd react hitting a river from 30 feet up, how much time would the occupants REALLY have, how could they REALLY get out... etc.
Most good Science fiction are simply stories about people reacting to their environment within the bounds of their humanity, and the constraints of known science.
That environment can be trumped up with constructs which are not explained... whether its faster than light travel, or an alien race governed by a hive mind... or it can be entirely mundane (as in Gravity or my imagined car accident story).
What makes it science fiction is that once the rules of the environment is established, the characters react to it constrained by the rules of science.
What separates good science fiction from fantasy is that fantasy is not bound to establish and then follow a set of physics. It's free to continually introduce whatever capabilities the characters need as the story needs it. Fantasy follows whatever path the author wishes without constraint. Science fiction's defining characteristic is that the narrative is constrained and driven by known physics or known or speculative physics.
Now you might say, but that's true of James Joyce's Dubliners; it too is constrained by the rules of phyiscs. None of the characters are magical or fantastical and nothing impossible according to known physics happens. And that's true. The difference between science fiction and ordinary (non-fantasy) fiction is that in science fiction the narrative is driven in part by the science. Dubliners narratives are not driven by science.
So even CSI could have been really good science fiction. Except its not, because despite the trappings of science they toss it out the window left and right. Star Trek with its particle-du-jour ... often is science fiction, because you are allowed to "pre-suppose" an alternate physics -- the trick is to play out the rest of the story constrained by it. Star Trek of course, as often as not, also fails to follow the rules it sets out for itself, and so deviates to space-fantasy or something... but many of its good episodes are good SF.