SMGW is used to monitor the gateway process. Gateway is used to communicate between SAP and NON-SAP systems. There will be only 1 gateway/ instance. If required we can also install a standalone gateway on a JAVA engine.
Gateway listens on port '3300'+instance number (3300, 3301,3302 where 01, 02 are the instances).
Gateway provides an interface so that the external system can communicate with SAP system on the specified port.
When RFC's are defined between the systems they use SAP gateway when (ALE, EDI, IDOC are transferring they use gateway)
You can also get information regarding SAP BASIS