How to use the gpxpy.gpx.GPXTrack function in gpxpy

def path_to_gpx(path, filename=None):
    gpx = gpxpy.gpx.GPX()

    # Create first track in our GPX:
    gpx_track = gpxpy.gpx.GPXTrack()
    gpx.tracks.append(gpx_track)

    # Create first segment in our GPX track:
    gpx_segment = gpxpy.gpx.GPXTrackSegment()
    gpx_track.segments.append(gpx_segment)

    gpx_segment.points = [(gpxpy.gpx.GPXTrackPoint(lat, lon, time=time)) for (lat, lon, time) in path]

    if filename:
        with open(filename, 'w') as gpx_fh:
            gpx_fh.write(gpx.to_xml())

    return gpx

def write_gpx(path, data):
    gpx = gpxpy.gpx.GPX()
    gpx_track = gpxpy.gpx.GPXTrack()
    gpx.tracks.append(gpx_track)
    gpx_segment = gpxpy.gpx.GPXTrackSegment()
    gpx_track.segments.append(gpx_segment)
    for point in data:
        gpx_segment.points.append(gpxpy.gpx.GPXTrackPoint(
            point[1], point[2], elevation=point[3], time=point[0]))
    with open(path, "w") as f:
        f.write(gpx.to_xml())

Point -> add as a Way Point
        LineString -> add all Points in a Route
        Polygon -> add all Points of the external linering in a Route
        Collection (of LineString or Point) -> add as a route, concatening all points
        """
        if isinstance(geom, GeometryCollection):
            for i, g in enumerate(geom):
                self.geomToGPX(g, u"%s (%s)" % (name, i), description)
        elif isinstance(geom, Point):
            wp = self._point_to_GPX(geom)
            wp.name = name
            wp.description = description
            self.gpx.waypoints.append(wp)
        elif isinstance(geom, LineString):
            gpx_track = gpxpy.gpx.GPXTrack(name=name, description=description)
            gpx_segment = gpxpy.gpx.GPXTrackSegment()
            gpx_segment.points = [self._point_to_GPX(point, klass=gpxpy.gpx.GPXTrackPoint) for point in geom]
            gpx_track.segments.append(gpx_segment)
            self.gpx.tracks.append(gpx_track)
        elif isinstance(geom, Polygon):
            self.geomToGPX(geom[0], name, description)
        else:
            raise ValueError("Unsupported geometry %s" % geom)

def write_gpx(path, data):
    gpx = gpxpy.gpx.GPX()

    gpx_track = gpxpy.gpx.GPXTrack()
    gpx.tracks.append(gpx_track)

    gpx_segment = gpxpy.gpx.GPXTrackSegment()
    gpx_track.segments.append(gpx_segment)

    for point in data:
        if len(point) > 3:
            gpx_segment.points.append(gpxpy.gpx.GPXTrackPoint(
                latitude=point[1], longitude=point[2], elevation=point[3], time=point[0]))
        else:
            gpx_segment.points.append(gpxpy.gpx.GPXTrackPoint(
                latitude=point[1], longitude=point[2], time=point[0]))

    with open(path, "w") as f:
        f.write(gpx.to_xml())  # loosing milliseconds here

def extract_segment_from_gpx_file(content, segment_id):
    gpx_content = gpxpy.parse(content)
    if len(gpx_content.tracks) == 0:
        return None

    track_segment = get_gpx_segments(
        gpx_content.tracks[0].segments, segment_id
    )

    gpx = gpxpy.gpx.GPX()
    gpx_track = gpxpy.gpx.GPXTrack()
    gpx.tracks.append(gpx_track)
    gpx_segment = gpxpy.gpx.GPXTrackSegment()
    gpx_track.segments.append(gpx_segment)

    for point_idx, point in enumerate(track_segment[0].points):
        gpx_segment.points.append(
            gpxpy.gpx.GPXTrackPoint(
                point.latitude, point.longitude, elevation=point.elevation
            )
        )

    return gpx.to_xml()