Director Alfonso Cuarón has denied the theory, stating that Gravity is meant to be a simple tale of survival and rebirth in the face of adversity, but he has acknowledged that the theory is intriguing.
Dr. Ryan Stone, the film's protagonist, is seen attempting suicide by hypoxia midway through the film to avoid a painful death, inside the salvaged Russian vessel, the Soyuz Capsule. It has already been established that the scientist has been deprived of oxygen for extended periods of time, having breathed in considerable amounts of carbon dioxide in her space suit after it ran out of oxygen. However, after hallucinating her dead partner, astronaut Matt Kowalski, Stone decides that she is not ready to die and figures out how to get through the atmosphere in the Chinese station, the Tiangong. She barely avoids drowning after the capsule lands in a secluded lake oasis, but does manage to swim to shore and take her first shaky steps on land, still adjusting to the Earth's gravity.
However, this does leave open the possibility that she never actually made it to Earth, but rather died in the Soyuz and either hallucinated finding paradise as she passed out, or made it into the afterlife. This ending is similar to what happened in the 1990 film, Jacob's Ladder. This makes sense because earlier in the film Ryan mentions that she was unable to get over the sudden death of her daughter, and later her hallucination of Kowalski directly tells her to "let go", meaning that she must let go of her guilt and trauma in order to move on with her life.
Also Ryan's hallucination of Matt, her resolve to carry on, her speech on the reentry and the lucky landing play out like an absolution. In other words, what should have happened rather than what actually did happen. Nonetheless, both versions of the story are realistic enough. Stone's hallucination of Kowalski is just as plausible as if he were a spirit advising her to come to terms with her daughter's death and move on into the afterlife. It could either be a journey to Earth, or a journey into the soul, but either way, she had personal business to wrap up in order to get there.