Populate a hidden field in Gravity Forms before submission

populate_gravity_forms_pre_submission.php

<?php
/* gform_pre_submission will do all forms. gform_pre_submission_1 will do a form with an ID of 1
*  Keep an eye on the priority of the filter. In this case I used 9 because the Salesforce plugin we used ran a presubmission filter at 10 so we needed this to happen before it
*/
add_filter( "gform_pre_submission_1", "add_salesforce_campaign_id_footer", 9 );
function add_salesforce_campaign_id_footer( $form ){
    foreach($form["fields"] as &$field)
        if($field["id"] == 2){
                    /* Set the variable you want here - in some cases you might need a switch based on the page ID.
                    * $page_id = get_the_ID();
                    */
                    $campaign_id = '701200000004SuO';
                    /* Do a View Source on the page with the Gravity Form and look for the name="" for the field you want */
                    $_POST["input_2"] = $campaign_id;
        }
    return $form;
}
?>