Can I override a CPT template on a per post basis with Gutenberg block editor?

Since 4.7 Custom Post Types have supported custom templates, just like Pages. So you could copy your theme’s generic page template to a new template file, and add this to the top.

<?php
/*
Template Name: Block editor template
Template Post Type: my-post-type
*/

That will let you switch the template for your my-post-type post to a template that looks and works just like a normal page.