An interesting, if unhelpful question. If Peter, James, and John fell asleep, who heard what Jesus prayed?
There were no videos or mobile phones to record with. There was not a scribe or stenographer to write down every word that Jesus spoke. No newspapers or investigative reporters and no editor to double check or verify facts. Gospels do not conform to any form of literature be it journalism or historical record
Having said that, it seems clear from how Jesus is portrayed across all the Gospels that He knew His path. The Garden of Gethsemane shows the humanity we need to see in Christ. In theory, He not only knew about His death but also His resurrection. As such he could have been cold and calculating and lacking all human emotion, but for it to matter He had to really live it. It is called “The Passion” for a reason.
Part of the reason for Christ coming to earth is to demonstrate empathy and understanding of the human life (including suffering and death).as well as sovereignty over all of it.
There is still faith in Christianity that supercedes the written words of the bible. The bible is the start, not the end of understanding.
Richard