Apple's App Store has a number of GPX viewers available for download to your iPhone. Some free, some paid. One of our favourites is GPX Navigator, which is £1.49 and can import GPX files directly from the website to your phone. Other apps are available. Just search GPX in the App Store.
Google Play has a number of apps that can display GPX routes on Android devices. A popular app is GPX Viewer which can display the GPX route on a Google map and allow it to be used for navigation.
Most routes were recorded during an actual walk. A user may have forgotten to start their GPS or phone at the beginning of the walk or may have taken a wrong turn and retraced their steps. This will be recorded in the GPX file. The routes shouldn't be used solely to guide you but, instead, used in conjunction with paper maps, handheld GPS or other navigational aids. The routes are a suggested basic route in any given area and should be treated as such.
These routes were gathered mainly by walkers. There's no reason though why they couldn't be ridden on bikes or horses if the trails are permissive (i.e. bridleways, BOATS, RUPPS, etc.). Check on an OS map to find out.