See the code sample here.

add_action('post_grid_loop_bottom', 'post_grid_loop_bottom_20200614', 10, 2);

function post_grid_loop_bottom_20200614($args, $post_grid_wp_query){
    $grid_id = $args['grid_id'];
    $post_grid_options = $args['options'];

    ?>
    <div class="item item-52833 skin flat even 0 ">Custom HTML here</div>
    <?php
}

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