Hello Wendy,
The Fellsman does not follow a specified route, so there is no definitive .gpx file. Apart from a few sections which are specified by the organisers, entrants are expected to plot their own route.
Most follow a pretty similar route , particularly in the first few sections. A good starting point might be to look at the opentracking website, which has the all of the entrants tracks on from last year-
The Fellsman 2022. Most of the minor variations in choice of route will be over the section between Redshaw and Park Rash (which for most mere mortals will be the night section!)
I last did the event in 2019 and am looking to enter again this year. I can also let you have a copy of my gpx track from 2019 if that would be any use.