gform_post_payment_transaction

add_action( 'gform_post_payment_transaction', 'my_function', 10, 6 );

function my_function() {
    // Do something here
    // If data from the Stripe webhook event is needed (and this hook was initiated via a Stripe webhook request), you can access event data with the following line:
    $event = gf_stripe()->get_webhook_event();
    if ( $event ) {
       // Access webhook event data. For event object documentation, see: https://stripe.com/docs/api/events/object
    }
}
add_action( 'gform_post_payment_transaction', 'my_function', 10, 6 );

do_action( 'gform_post_payment_transaction', $txn_id, $entry_id, $transaction_type, $transaction_id, $amount, $is_recurring );