GPSBabel is a comprehensive FOSS conversion utility, with a customizable list of converters, which sadly is not natively portable -- the download package can be easily (Uni)extracted (discard both the 'Additional files' folder and the 'install_script.iss' file), and it will run without issues, although it will save its settings to 'HKCU\Software\GPSBabel'.
FYI, an old Portable Apps Format package is still available from https://sourceforge.net/projects/gpsbabelportabl/, which might be enough for some limited purposes.
- [url]https://www.gpsbabel.org/[/url] author wrote:GPSBabel converts waypoints, tracks, and routes between popular GPS receivers such as Garmin or Magellan and mapping programs like Google Earth or Basecamp. Literally hundreds of GPS receivers and programs are supported. It also has powerful manipulation tools for such data. such as filtering duplicates points or simplifying tracks. [...] By flattening the Tower of Babel that the authors of various programs for manipulating GPS data have imposed upon us, GPSBabel returns to us the ability to freely move our own waypoint data between the programs and hardware we choose to use.