Depends what you mean by viewing the GPS file.
If you want to look at the actual raw data, then you can just open it in Notepad or other text editor. Handy if you want to edit your Strava data......
Or to view the route graphically, just import it into a website like bikehike or RidewithGPS.
Regarding Sustrans routes, their map shows which bits are either on or off road. Some sections of off road will be smooth tarmac, but most won't be.
https://www.sustrans.org.uk/ncn/map