Mainly for reference, maybe it's useful for someone else:
gpsbabel -T -i nmea -f /dev/cu.usbmodem3d11 -F gps.csv
and it prints the coordinates to the output. gpsbabel has many output formats, with eg.
gpsbabel -T -i nmea -f /dev/cu.usbmodem3d11 -o kml,points=0,line_color=640000ff,max_position_points=10 -F gps.kml
you get a KML file, which you can open with Google Earth.
Google Earth is by the way also how I found out the needed parameters, since it "just worked" there. I started Google Earth and its Realtime GPS feature and looked at the process list with "ps aux" and got, what I needed (especially the "/dev/cu.usbmodem3d11" parameter may be different on your Mac)