Sum total of all fields with a class of XXXX and throw a validation error if the sum is == 0

iso_gf_min_sum_validation.php

<?php

//Don't forget to change the _1 in the hook to the ID of your form
add_filter('gform_validation_1', 'iso_min_sum_validate');


// Most code taken from http://www.gravityhelp.com/documentation/page/Using_the_Gravity_Forms_%22gform_validation%22_Hook
function iso_min_sum_validate($validation_result) {

    // We'll base our loop on CSS class
    $css_class_we_want = 'product-quantity';

    $form = $validation_result["form"];

    // Set up a counter for our field
    $my_total = 0;

    // Loop to see if we have a total greater than 0
    foreach($form['fields'] as &$field){

        // 5 - If the field does not have our designated CSS class, skip it
        if(strpos($field['cssClass'], $css_class_we_want) === false)
            continue;

        $my_total = $my_total + intval(rgpost("input_{$field['id']}"));
    }


    if($my_total == 0) {
            foreach($form['fields'] as &$field){

            if(strpos($field['cssClass'],  $css_class_we_want) === false)
                continue;

            $field['failed_validation'] = true;
            $field['validation_message'] = __('Please select at least 1 product', 'pla');
        }
        
        $validation_result['is_valid'] = false;
        $validation_result['form'] = $form;
    }

    return $validation_result;
}