Gateway credentials are not verified when the gateway is created. If you are looking to verify whether a gateway's credentials were created properly, you can attempt a real purchase transaction on the live gateway (not a test transaction) using Test Data.
If your credentials are correct the error message you get from the gateway should be related to the card (since the card numbers are not chargeable in a production environment). If you get an error message about credentials, that is a sign that you need to update the gateway and try again.