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
}

 

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