That is good information. I recommend adding some additional logging to the form, since you already have logging enabled. Add this to your theme functions.php file:
add_filter( 'gform_paypal_query', 'log_the_paypal_query', 10, 3 );
function log_the_paypal_query( $query_string, $form, $entry ) {
GFCommon::log_debug( __METHOD__ . '(): This is the query string => ' . print_r( $query_string, true ) );
GFCommon::log_debug( __METHOD__ . '(): This is the entry => ' . print_r( $entry, true ) );
return $query_string;
}
That will log the query string and the entry and we can see exactly what is being sent and if there is a way to change that. This information will be logged to the Gravity Forms Core log after you test the form.