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.
Sakamoto wasn't the main character of an anime. He was a ninja in Japanese history, and was known as the " God of Ninjas."
If one had to say that the male lead of the anime was more pretentious than Sakamoto, it might be Tanjiro from Ghost Blade. Tanjiro was an ordinary person, but he always displayed amazing strength in battle. It could even be said that he was pretentious. He constantly challenged strong enemies to improve his skills and was not afraid of any difficulties or challenges. His image was very much in line with the definition of the word 'pretentious'.
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.