You can force an ACF custom block to show each new Page that is created by setting up a Gutenberg template that includes the block you’d like, with this code:
function pages_guten_template() {
$post_type_object = get_post_type_object( 'page' );
$post_type_object->template = array(
array( 'acf/custom-block-name' ),
);
}
add_action( 'init', 'pages_guten_template' );
More details on templates can be found here (including how to lock it in place, if you so desire).
