set up pipenv with supervisor

You need to give explicit path of gunicorn. Though I'm not sure about pipenv, but the error you are getting is because supervisor tries to find gunicorn in the directory. You should change your config file into something like this:

[program:back]
directory=/home/dev/app/
command=/path/to/pipenv run /path/to/gunicorn config.wsgi:application --bind 127.0.0.1:8005

Then you must restart your supervisord in order to load the settings.

sudo service supervisord reload