add_action('post_grid_main', 'post_grid_main_20200614', 10);
function post_grid_main_20200614( $atts){
$grid_id = $atts['id'];
$post_grid_options = get_post_meta( $grid_id, 'post_grid_meta_options', true );
?>
<div class="">Custom HTML here</div>
<?php
}