GD Mail Queue Pro plugin has support for custom SMTP services for PHPMailer. One of the services included is for the SparkPost email sending service. And, to start using this service to send emails using SMTP you need SparkPost account and make sure that you SMTP option activated. After you have set up your SparkPost account, you need to get SMTP access information to add to the GD Mail Queue Pro plugin SparkPost Service settings panel.
To get the required information, open your SparkPost account, make sure that you verify the domain you plan to use for sending emails, and enable Send via SMTP option. Once you enable this option, you will get the API key, and that key is used as a password in GD Mail Queue Pro plugin.