To add a new Payment Gateway connection in Chargent, simply create a new gateway record from the "Gateways" tab, with the data filled out as detailed here: http://www.appfrontier.com/documentation.html#gateway
You will need credentials from your payment gateway, and to check how those credentials map to the Chargent gateway fields:
Merchant ID Merchant Security Code Merchant Reference (only used in some gateways) Transaction Security Key (only used in some gateways)
Once you have created the gateway record, you will want to run some tests.
Set the "Test Endpoint" field = True and some test cards provided by your payment gateway, to make sure the connection is working. Note that some gateways have separate credentials for test environments and some do not.
Once it is working be sure to uncheck that field to send to the live service.
If you have more than 1 active gateway, you will need to define which gateway to use in each Chargent Order record (if only one gateway is active then it defaults to that one).