add_filter('gform_validation_9', 'validate_donation');
function validate_donation($validation_result) {

    //if form has already failed validation, don't process donation
    if(!$validation_result["is_valid"]){
        return $validation_result;
    }

    // Make a call to process_payment function to process payment and check validity
    $is_valid = process_payment();

    // If the payment is valid don't do anything, return $validation_result
    if($is_valid){
           // Return the validation result
        return $validation_result;
        }
    else{
        // Fail the validation for the entire form
        $validation_result['is_valid'] = false;
           // Return the validation result
        return $validation_result;
        }
}

add_filter("gform_validation_message_9", "change_message", 10, 2);
function change_message($message, $form){
    global $donate_error;

    if(!empty($donate_error)){
        //only display custom validation message if there was an error with the donation processing
        return '<div id="donate_error">' . "An error occurred while processing your submission.<br/>Please modify your information based on the error below and try again.<br/><br/>" .  '<span>' . "Error: {$donate_error}" . '</span>' . '</div>';
    }
    else{
        //display default message when a standard form validation error occurs.
        return $message;
    }
}