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
}