Add Class to Gravity Form Hidden Fields

functions.php

<?php

add_filter( 'gform_field_input', function ( $input, $field, $value, $lead_id, $form_id ) {

    if ( $field->inputName !== '' && $field->type == 'hidden' ) {
		$input = "<input type='hidden' id='input_{$form_id}_{$field->id}' name='input_{$field->id}' class='{$field->inputName}' value='{$value}'>";
    }

    return $input;

}, 10, 5 );