This is how I do it. I have had no difficulty at all when using it this way.
Download Tyretotravel software. Basic features are free to use. That is enough to download GPx files and upload the files in to your garmin or TomTom. I use the basic free one.
Once you install it in your PC, and download any GPx file, your PC will automatically select that programme to open GPx files.
If it doesn't and keep asking you how to open, ask the PC to use tyre.
There are very good YouTube clips if you are stuck on how to use Tyretotravel.
But I think tyre is not available as an app to use on your smartphone. For that use GPx viewer as suggested. It works fine.