Rungap for iOS exports GPS data as JSON file in the free version. This script converts it to GPX. Tested with python3.6. Might need to install required modules. Simply place either the metadata and data json files, or the complete zip file in the same directoryas the script and run it. Warning: Does barely any error checking

RungapGPX.py

import xml.etree.cElementTree as ElementTree
import json, pytz, zipfile, unicodedata, re
from datetime import datetime
from os import listdir
from os.path import isfile, join
import glob

def slugify(value):
    """
    Normalizes string, converts to lowercase, removes non-alpha characters.
    Slightly modified from https://github.com/django/django/blob/master/django/utils/text.py
    """ 
    value = str(value)
    try:
        value = unicodedata.normalize('NFKD', value).encode('ascii', 'ignore').decode('ascii') 
        value = re.sub(r'[^\w\s-]', '', value).strip().lower()
        value = re.sub(r'[-\s]+', '-', value)
    except:
        print('Could not slugify filename, will use default one instead')
        value = 'output'
    return value

creator = "GapX"
version = "1.1"

zip_files = glob.glob("*.zip")

for zip_file in zip_files:
    print("Unzipping " + zip_file)
    zip_ref = zipfile.ZipFile(zip_file, 'r')
    for info in zip_ref.infolist():
        print(info.filename)
        if info.filename.endswith("metadata.json") or info.filename.endswith("rungap.json") or info.filename.endswith("nike.json"):
            zip_ref.extract(info)
    zip_ref.close()

metadata_files = glob.glob("*metadata.json")
data_files = glob.glob("*rungap.json")
data_files += glob.glob("*nike.json")
print("Found metadata files")
print(metadata_files)
print("Found data files")
print(data_files)

if len(metadata_files) != len(data_files):
    print("Error, number of metadata files does not match number of datafiles!")
    exit(1)


for idx, metadata_file in enumerate(metadata_files):
    print("=========")
    print("Parsing " + metadata_file + " and " + data_files[idx])
    metadata_data = json.load(open(metadata_file))
    data = json.load(open(data_files[idx]))

    root = ElementTree.Element("gpx")
    root.set("xmlns","http://www.topografix.com/GPX/1/1")
    root.set("creator", creator)
    root.set("version", version)
    metadata = ElementTree.SubElement(root, "metadata")

    name = metadata_data["title"]
    desc =  metadata_data["description"]
    print("Route name: " + name)
    print("Route description: " + desc)
    ElementTree.SubElement(metadata, "name").text = name
    ElementTree.SubElement(metadata, "desc").text = desc
    ElementTree.SubElement(metadata, "time").text = metadata_data["startTime"]["time"]
    timezone = pytz.timezone(metadata_data["startTime"].get("timeZone", "UTC"))
    source = metadata_data["source"] + " exported by Rungap for iOS, version " + metadata_data["appversion"]
    
    track = ElementTree.SubElement(root, "trk")
    ElementTree.SubElement(track, "name").text = name
    ElementTree.SubElement(track, "desc").text = desc
    ElementTree.SubElement(track, "src").text = source
    segment = ElementTree.SubElement(track, "trkseg")
    if "laps" in data:
        print("Found " + str(len(data["laps"][0]["points"])) + " track points")
        for point in data["laps"][0]["points"]:
            if ("lat" in point and "lon" in point and "ele" in point and "time" in point):
                trkpt = ElementTree.SubElement(segment, "trkpt", lat=str(point["lat"]), lon=str(point["lon"]))
                ElementTree.SubElement(trkpt, "ele").text = str(point["ele"])
                ElementTree.SubElement(trkpt, "time").text = datetime.fromtimestamp(point["time"], timezone).isoformat()
    elif "laps" in metadata_data:
        print("Found " + str(len(metadata_data["laps"])) + " track points")
        for point in metadata_data["laps"]:
            p = point.get("startLocation", {})
            if ("lat" in p and "lon" in p and "startTime" in point):
                #dt = datetime.datetime.strptime(d, "%Y-%m-%dT%H:%M:%SZ")
                trkpt = ElementTree.SubElement(segment, "trkpt", lat=str(p["lat"]), lon=str(p["lon"]))
                ElementTree.SubElement(trkpt, "time").text = point["startTime"]
        
    gpx_filename =  slugify(name + " - " + datetime.now().isoformat()) + ".gpx"
    print("Writing " + gpx_filename)
    tree = ElementTree.ElementTree(root)
    tree.write(gpx_filename, "UTF-8", True)

Ah, I think I know what the issue was. Should work with the zip file now too