If needed, Guitar Pro can export its latest .gpx files to the previously used GP5 format (version 5), which is what gpx to gp5 conversion typically represents. You can do this easily from within of the export menu, however this option might not be available in the trial / lite version of the program. As for 3rd party solutions or other alternative gpx to gp5 converters, no such thing exists.
Open GPX file Open GP5 file