<?php
add_filter("gform_pre_render_".FORM_EDIT_POST, "populate_category_checkbox");
/**
* Pre populate category chackbox fields frontend.
*/
function populate_category_checkbox($form){

    if( $field["type"] == 'post_category' ){

        $categories = get_the_category( $postid );
        if( $categories ){
            foreach( $categories as $category ) {
                $selected_cats[] = $category->name;
            }
        }
        ?>
        <script type="text/javascript">
            jQuery(document).ready(function($) {
                jQuery(document).bind('gform_post_render', function(){
                    var selected_cats = <?php echo json_encode($selected_cats); ?>;
                    var cat_container = $('.select-category');
                    $('.select-category ul label').each(function(index, value) {
                        if($.inArray($(this).text(), selected_cats) > -1) {
                            $(this).prev().attr('checked', 'checked');
                        }
                    });
                });
            });
        </script>
        <?php
    }

    return $form;
}