Want to use post-Id as unique key

I’d like to auto-generate a unique item number and am thinking the post-id would more than adequate, and a new one is created for each post/page.

My guess on how to do this would be to create a template from an existing one, and put it in my child theme. I like to know the general steps to grab a copy of the post-id and insert it into an item number field in the post.

I have Gravity Forms and it would be cool to be able to do this in one of the form fields.

Alan R.