Follow the directions here to create a client_id.json file containing authentication credentials for your google calendar account. Place the file in the same directory as the gcal.py script.
Some basic settings are in the gcal.cfg file, which you can edit to specify a different sound file or calendar.
The first time you run it, use python gcal.py --noauth_local_webserver and follow the instructions.
Subsequently, run with python gcal.py.