I would suspect this has to do with timezone data. In the live caches Cachly gets the timezone from the API for all caches and it knows how to correctly convert the timestamp. If you import a GPX, there likely is no timezone data and Cachly just uses UTC.
The navigation provider can be chosen in settings, and you can choose Apple. Google, Waze or TomTom. Once you do that, tapping "Navigate" will use that mapping app.
Is that what you were asking?