Where is the Gunicorn config file?

The answer is in the documentation of gunicorn.
http://docs.gunicorn.org/en/latest/configure.html

You can specify the config file with .ini or a python script.

For example, from the django-skel project

"""gunicorn WSGI server configuration."""
from multiprocessing import cpu_count
from os import environ


def max_workers():    
    return cpu_count()


bind = '0.0.0.0:' + environ.get('PORT', '8000')
max_requests = 1000
worker_class="gevent"
workers = max_workers()

And you can run the server using

gunicorn -c gunicorn.py.ini project.wsgi

Note that project.wsgi correspond to the location of your wsgi.