When Chargent is installed in a Salesforce Sandbox, transactions are always sent to the payment gateway's test / sandbox / development environment, regardless of whether Chargent's Test Endpoint checkbox is checked. This is done as a security precaution, to prevent real transactions from being accidentally sent from a Salesforce Sandbox.
When Chargent is installed in a production or developer Salesforce org, the Test Endpoint checkbox on the Gateway record will select between sending to the payment gateway's live and test environments. (There are a few exceptions, such as Stripe, which has a single endpoint to send transactions to, and different credentials for live versus test transactions).
If you wish to send live transactions from a Salesforce Sandbox, as a final step in testing, simply use Chargent's Endpoint Override field on the Gateway record, and enter the production endpoint of your chosen gateway there.
Here are the production endpoint URLs for Chargent's integrations:
Authorize.net AIM https://secure.authorize.net/gateway/transact.dll
Authorize.net CIM https://api2.authorize.net/xml/v1/request.api
Barclays ePDQ (Ogone) https://payments.epdq.co.uk/ncol/prod/orderdirect.asp
Chase Orbital https://orbital1.paymentech.net/authorize
Litle / Vantiv https://transact.litle.com/vap/communicator/online
Merchant e-Solutions https://api.merchante-solutions.com/mes-api/tridentApi
Moneris (CA) https://www3.moneris.com/gateway2/servlet/MpgRequest
Network Merchants NMI https://secure.networkmerchants.com/gateway/transact.dll
PayFlow Pro https://payflowpro.paypal.com
Note that the full endpoint URL must be entered in the Endpoint Override field, and the domain must be present in the Remote Site Settings (for standard Chargent integrations it should be already present).