Apologies for late reply.
So what I did was I added this line of code:
‘show_in_rest’ => true,
in plugin’s index file in array section of this function:
function cp_create_post_type() {
(in my case line 141). This I think is what connects (allows) Gutenberg. It works for me without renaming any templates.
Hope this will help. And thank you for your assistance.