When setting up your form, you need a way to tell Gravity Forms which feed to process.

This is my recommendation:

1. Add a Payment Method radio button to your form with two options: Credit Card, eCheck
2. Enable Conditional Logic in your Credit Card and eCheck fields and set your Credit Card field to be conditional upon the Credit Card choice and eCheck upon the eCheck choice (Advanced Tab on your field)
3. In your feed settings, enable Conditional Logic and set the condition based upon the customers choice of payment as well
4. You need to go through this with both the Credit Card and eCheck fields so that only one feed is processed at a time.