See the code sample here.
add_action('post_grid_item_layout', 'post_grid_item_layout_20200614', 10);
function post_grid_item_layout_20200614($layout_args){
$layout_id = $layout_args['layout_id'];
$post_id = $layout_args['post_id'];
$options = $layout_args['options'];
?>
<div class="">Custom HTML here</div>
<?php
}