The Mexican filmmaker wrote and directed the movie, which stars George Clooney and Sandra Bullock as astronauts caught in a crisis in space.
Downey Jr was originally to star in the film, with Natalie Portman in talks, but delays in production forced both actors to drop out.
The Huffington Post quotes Cuarón saying at Comic-Con: "Downey was attached. But we just kept on extending the process. And also it was very clear that the technology we were going to use - it was not the most compatible thing for what Robert is the best at.
"That is, he takes one scene and he just starts riffing. And because of the technology that we use, it's pretty much limited. We have to pre-programme the film before shooting."
Gravity, set to open the Venice Film Festival, is due for release in October 2013.
Watch a trailer for Gravity below:
Photo gallery - Comic-Con 2013: