Basically you have two options:
- Use https://myopenhab.org/gpstracker/owntracks which will proxy requests to /gpstracker/owntracks. In this case the authentication is handled by myopenhab.org
- You can set up a reverse proxy with Nginx. In this case you can choose the port you like (https://<your.ip.address>:PORT/gpstracker/owntracks) and you need to configure the authentication as well in Nginx. Here is a community topic with this subject.