I read that Gihon is actually the Nile and I was wondering how that’s physically possible considering that the Nile flows from Lake Victoria which isn’t remotely close to the Tigris or the Euphrates.
The Ben Ish Chai writes in the name of the Arizal that Gan Eden is not composed of physical matter similar to the rest of our world, but something much more refined. He also uses this to explain how the Gemara in Maseches Pesachim 94a could say Gan Eden is sixty times the size of the world: since it is of a very holy and refined matter, it does not take up physical space as we perceive it.
With this understanding of Gan Eden, the pesukim referring to a river leaving Gan Eden and turning into the four rivers we know of today cannot be referring to something we would be physically able to observe, just as we cannot observe actual Gan Eden.
(Interestingly, the Ben Ish Chai is very skeptical of scientific knowledge, and writes at length of its limitations. His understanding of Gan Eden would seem to be purely based on Torah sources.)