If you enabled the RDS Gateway or also a redundant gateway at the provisioning with the following slider:

You will get a task to upload SSL certificate in .pfx format after the provisioning is done. One SSL can be used for both Gateways.

The certificate can be exported from ANY server that has IIS installed on it. 

Once you have the cert in .pfx format, mark the task and click the check mark to upload it to the system.

The system will take the cert and configure your gateway with it automatically.

Pro Tip: To confirm your SSL was installed correctly, you can login to the the gateway server (RDG) and open IIS Management Console to view the certificates.