When using the PayPal Add-Ons for Gravity Forms, you can set various transaction types depending on your needs such as Products and Services, Subscription, and Donations.
Transaction Type-specific Options
If you select Donations from the Transaction Type drop down, the option below will appear.
The Payment Amount setting denotes which field will be used for the payment amount. It can be set to Form Total or an additional field that you have configured.
If you need more information on configuring feeds within the PayPal Payments Standard Add-On, you may review our article on Creating a Feed for the PayPal Payments Standard Add-On.