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
}

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