From what I understand a GPX file is a collection of points, lines and polygons. It is used to show a route on an Ordnance Survey map or other mapping system. If you have OS Maps or Anquet, Viewranger etc and have purchased OS tiles, the gpx file can be loaded and the particular application will use this to show the path of a complete route on the ordnance survey map.
If you use the OS Maps app and look for routes already set up, you can view them but they always show with a thick red line for the route, obscuring the details beneath of footpath, byways etc. It is impossible to change this as the route is not “your” route. However, if you take the option to ‘Export gpx’ then save it locally, close the route on the screen and take the option to Import gpx, it’ll open up as your route with the availability of an Edit option. At that point you can change the route itself by dragging waypoints, or amend the style of the route by colour, thickness or transparency.