For example if you are adding a Payment Gateway for onBoard follow the steps below:
Navigate to:
  1. onBoard > Settings > Payment Processing
  2. To add the payment gateway information, click the Add New Account button
  3. Select the payment gateway type in the Gateway drop down menu
  4. Mark the Active check box to activate the payment gateway account
  5. Enter the Account Login and Password information for the Account
  6. Select the Credit Card options if any that you will be using
  7. Create a Description for the account (i.e. Contracts)
  8. Click Save

If you are adding a Payment Gateway to onMessage for example, you would select Settings and then the Product such as Giving, Store, or Event Registration. The Payment Gateways are all used exactly the same in onMessage so if you add it to Store, or Giving etc then you would just select that Payment Gateway on that Form you are looking to add it to, you do not need to add the same Gateway multiple times for each Product.