See the code sample here.
add_action('post_grid_loop', 'post_grid_loop_20200614', 10);
function post_grid_loop_20200614($args){
$grid_id = $args['grid_id'];
$post_grid_options = $args['options'];
$post_id = isset($args['post_id']) ? $args['post_id'] : ''; ;
?>
<div class="item item-52833 skin flat even 0 ">Custom HTML here</div>
<?php
}