These are the instructions for batch converting NMEA formatted GPS file to GPX format on Windows 7. Download and install GPSBabel (http://www.gpsbabel.org/download.html) first.
Open a command prompt and switch to your directory of NMEA files. Mine are in:
for /f %i in ('dir /b *.txt') do "C:\Program Files\GPSBabel\gpsbabel.exe" -w -t -i nmea -f C:/Workspace/GPS/nmea/%i -o gpx -F C:/Workspace/GPS/gpx/%~ni.gpx
This can be translated as follows. For all the text files in the directory read them in as NMEA format and write them out to GPX format in the gpx folder. The modifier strips the .txt extension from the filename and writes out the input name with the .gpx extension.