Historically, John has always been established as the author of the gospel that bears his name.
In the second century, Irenaeus, one of the wrote extensively about John being the author of the namesake gospel. Irenaeus would have been familiar with the correct author of John because he was so close to the apostles and the events of the time.
As said previously, the Church Fathers were familiar enough with the scriptures to know if something was off about the text. It’s highly improbable they would set to memory text of which they were uncertain or, for that matter, consider it ‘gospel truth’.