You could try upgrading the timeout for your proxy pass in Nginx by adding:
proxy_connect_timeout 75s;
proxy_read_timeout 300s;
on /var/nginx/sites-available/[site-config] or /var/nginx/nginx.conf if you want to increase the timeout limite on all sites served by nginx.
You must add --timeout 300 as well to your gunicorn process/config.
This solved my problems in the past with bigger uploads.