No, gravity isn't a true story. It's a physical phenomenon described by laws and principles of physics. We understand it through experiments and observations, not as a narrative with characters and events.
Typically, the decisions about who lives, who dies, and who tells the story in an animatic are made by the team of writers and animators. They consider various factors such as character arcs, the overall theme, and the intended impact on the audience.
Gravity Falls was not based on a comic. It was a unique creation that emerged from the imagination of its makers, bringing a fresh and engaging storyline to the screen.