Display $pagenow error notice on all admin pages

I need to show my client error message on all admin pages.

I have the following code, that adds a custom notice only on the admin dashboard page:

add_action('admin_bar_menu', 'custom_toolbar_link2', 999);

function general_admin_notice(){
    global $pagenow;
    if ( $pagenow == 'index.php' ) {
         echo '<div class="notice notice-error">
             <h3>My custom text</h3>
         </div>';
    }
}

Is there any way to display this notice on all admin pages?

function my_admin_notice() {

    /*
     * The class of admin notice should be "notice" plus any one of
     * -"notice-error",
     * -"notice-warning",
     * -"notice-success"
     * -"notice-info".
     * Optionally use "is-dismissible" to apply a closing icon.
     */

    echo '<div class="notice notice-info"><p>Custom notice text</p></div>';

}

add_action( 'admin_notices', 'my_admin_notice' );

The only thing is, these notices does not show up on New Post, Edit Post and similar where Gutenberg reins. There must be the solution, but I can’t find it right now.