You can pass each settings file to guincorn separately for each project. I'd recommend looking into supervisord to manage your guincorn processes so that you can keep one running for each project with different settings.
You can set up your config file like
[program:project1]
command=/var/www/project1/manage.py run_gunicorn --settings=project1.settings.production
directory=/var/www/project1/
environment=PATH="/var/www/project1/venv/bin"
user=www-data
autostart=true
autorestart=true
[program:project2]
command=/var/www/project2/manage.py run_gunicorn --settings=project2.settings.production
directory=/var/www/project2/
environment=PATH="/var/www/project2/venv/bin"
user=www-data
autostart=true
autorestart=true
So instead of defining it in an environment variable, you can define your settings in your supervisord config file, which will then maintain the process for each of your projects.