Include a Gutenberg Block in a PHP file

You need to use Custom Post Type for blocks. What it does is you can register your own post template with predefined blocks in it. Which I believe is what you are looking for. Here’s example of that –

function myplugin_register_book_post_type() {
    $args = array(
        'public' => true,
        'label'  => 'Books',
        'show_in_rest' => true,
        'template' => array(
            array( 'core/image', array(
                'align' => 'left',
            ) ),
            array( 'core/heading', array(
                'placeholder' => 'Add Author...',
            ) ),
            array( 'core/paragraph', array(
                'placeholder' => 'Add Description...',
            ) ),
        ),
    );
    register_post_type( 'book', $args );
}
add_action( 'init', 'myplugin_register_book_post_type' );

This function registers a CPT and via template option it specifying which block are inside of it. You can lock, loosely lock or allow other blocks to be inserted as well.