Good question: what does Gravity have that Titanic doesn't? Both are survival tales that deal with what can go horribly wrong with the latest technology, but the eerily prescient Futility wasn't considered sci-fi at the time. I think it's a sign that we live in interesting times that the definition of sci-fi is getting blurry. Apollo 13 counts as historical drama despite having a very similar topic to Gravity, mostly because the events in Apollo 13 did actually happen. For comparison, The Prestige is classified as sci-fi despite occurring in our relative past, and Left Behind, although set in the future, is not sci-fi by any definition.