@Meysam, you can use the gateway-put API to add a gateway. The one that I created in the image below has the following properties:
address - IP/FQDN of gateway
authPassword - Authentication password, if required
authUsername - Authentication username, if required
description - Optional description for this gateway configuration
encryptionType - 0 = None, 1 = SSL, 2 = TLS
id - Must be null for new gateways
isEnabled - Whether or not this gateway is enabled
port - Port on which to connect to this gateway
shouldAuthenticate - True if authUsername and authPassword are required; otherwise, false
smPassword - Password if using SmarterMail Gateway mode
smUrl - URL if using SmarterMail Gateway mode
smUserName - Username if using SmarterMail Gateway mode
type - 0 = Round Robin, 1 = Specific domains