This is a core block and thus available by default in Gutenberg WordPress editor. No download required.
The WordPress More Block in Gutenberg is used to truncate the content for the homepage, archive pages or subscriber emails. It works the same way as the more tag in the classic editor. Most often this stuff is handled by the themes for all the archive pages. This may be a block you might never use, but still useful in some cases.
More Block Features
This a really simplest block and has no options in the block toolbar. You may change the more text by simply click and start typing on the Read More text. In the sidebar settings panel, it has option Hide the teaser before the more tag. If enabled, it hides the content that appears before more block on the full post.
More block added on a page or post.
More block sidebar settings panel.