-Open your nginx.conf file located in /etc/nginx directory

-Add this below piece of code under http { section

client_header_timeout 3000;
client_body_timeout 3000;
fastcgi_read_timeout 3000;
client_max_body_size 32m;
fastcgi_buffers 8 128k;
fastcgi_buffer_size 128k;

Note - If its already present , change the values according.

That’s it. Reload Nginx and php5-fpm.

service nginx reload

service php5-fpm reload

If the error persists ( which is rare ), consider increasing the values.

nguồn: http://www.codetweet.com/nginx/solved-504-gateway-timeout-error-in-nginx-web-server/