Visit http://www.tapr.org/~kh2z/Waypoint/ to download the excellent little Waypoint+ program written by Brent Hildebrand. This supports upload and download of waypoints and tracks between several models of GPS and several file formats.
GPSS v5.95 and later supports the conversion between GPSS GTX format and Waypoint+ comma seperated text file format.
Use Ctrl-C to switch on the experimental page of controls. You will see the two buttons to convert from a Waypoint+ .txt file to gpss.gtx and another button to plot this file.
GPSS Ctrl-A Analysis functions such as REPLAY and PLOT will now handle a Waypoint+ downloaded track file with name TR*.TXT. e.g. TRACK.TXT, TRKHOL2.TXT, etc.