I would still consider it. Even if it’s theoretically editable, it’s still evidence to back up your claim. Independent witnesses can be fabricated, too, but we usually believe them! I very much doubt anyone would present a falsified gpx file in court – wouldn’t this turn a simple civil offense hearing into perjury?
Also, the driver may not have a clue that gpx files are editable – so it may be enough evidence to persuade them to cough up even if it wouldn’t be strictly admissible in court.
I had an accident recently and I presented the Strava ride as evidence to the driver (along with, fortunately, a witness) and I got a settlement amount. I don’t know whether it made much difference but it all adds to the puzzle.