I’m the crestfallen organiser of this event. The riders do get a GPS but when we start visiting the singletrack the plethora of trails means in the plantations means that it is easy to be on the wrong track when you think you are on the correct one.
I didn’t intend my comment in a mean way. I’ve never been to the IOM, and dont know the terrain, nor am I familiar with Manx access issues, but similar sounding (50k+ single lap events) I have done in England have relied on GPX and arrows together, there are certain areas where arrows were insufficent alone (whether they were tampered with, or just poorly placed) and having a GPX, or being in sight of riders in front was necessary.
Of course, in England, these are very officially “not a race” due to laws about racing on public rights of way.