Board Thread:Questions and Answers/@comment-5919632-20130703035329/@comment-1739368-20130703044343

The story tends to focus alot more on Gyro, his actions, hardships and past quite a bit more than Johnny. He was also introduced first, during the point when Steel Ball Run wasn't officially announced as part of the JoJoverse yet, so many believed he was the main character.

Simply put, if someone who wasn't familiar with the whole "JoJo" thing read it, they'd assume Gyro was the main character. I would say the two share the main protagonist spot, as it says on Gyro's page.