The current version of the Gutenberg editor does not give us a way to add the Excerpt Delimiter directly in your post, unfortunately.
You can click the Document tab (on the right side) and scroll down to the Excerpt widget. If you like to do it that way great, but personally, I don’t want to have to re-type the beginning of my article in that little box. Plus it looks like you can’t enter but plain text (I did not test, so really I do not really know of the capabilities.)
The way to do it in the current version (I really hope that the feature will soon be added to the editor) is to create a paragraph between two paragraphs, where you want the cut off to be.
Start typing something in that paragraph. Without anything there, the editor does not give you the option to edit the paragraph as HTML (yeah, double the trouble!)
Once you have some text (one letter is enough), the mini-toolbar appears right above your paragraph and you can finally click on the trouble dots and select:
Edit as HTML
Now you can delete the text you entered and replace it with the following:
<!--more-->
This is called an HTML comment. A comment starts with <!-- and ends with -->. Whatever is inside a comment is ignored by browsers, so it can safely appear on a page and not cause any problems.
Internally, WordPress will recognize that comment and know that whatever appears before it is the excerpt and whatever is after is the body of your post. That is, you can make the excerpt not appear as part of your post.
Try this:
Click outside of the new block used for the excerpt. The editor asks you whether to switch back to the Visual Editor (instead of HTML). Click on that button. Now click on that block. The editor shows that the block in a “more” block.
In the settings, you have one option which is to tell whether the Excerpt should be part of the post or not (see Hide the teaser before the “More” tag on the image). In other words, whether to show it to people who visit the page itself and in other places like your front page. In my case, I like to have the beginning of my article as the Excerpt because then it makes sense to people, if they are interested and click the Read More or Continue Reading link under the Excerpt, they can continue reading instead of starting over from the beginning of the article.
Many say that the Excerpt should be a resume of the post, I think that the title of the post is already a resume and that’s enough for the user to decide whether to read that post or not.
Anyway, here we are. There is a way to get the separator, it just requires us to be a bit technical until it gets fixed in the editor. Hopefully soon so we don’t have to waste that much time just to add the READ MORE delimiter by hand each time.