From the Home page:
Tools | Track Log | Menu | Setup | Track Interval
gives you the options of:
- Automatic
- Time (with a user-settable time step)
- Distance (with a user-settable distance step)
I believe the GPX format just stores a list of Height/Lat/Lon/Time quartets,
and doesn't care about any frequency.
I have always used "Automatic", and the step size typically varies from 15 seconds when I am flying straight to 4 seconds when I am turning.