the error message is self explaining - your user 'user' connecting from the host @'xxxx.xxxx.xxxx' is not allowed to log into the MySQL database. To solve this issue you need to grant permissions to the user so that he can connect from any host (%) or just from that particular host where you installed the gateway.
The command to allow access from everywhere would be:
grant all on *.* TO '<user>'@'%' IDENTIFIED BY '<password>';
or just specify the IP address instead of '%' if you want to restrict it.
More details can be found in the MySQL manual: