This is a core block and thus available by default in Gutenberg WordPress editor. No download required.
The WordPress Separator Block in Gutenberg adds a separator between your content simply. It adds visual separator which may be a solid line or dotted line depends on the style you select.
Separator Block Features
The separator block toolbar has no options. In the block sidebar panel, there is a style picker which has three styles currently.
This is how the separator looks on the frontend.
The separator block in the editor.
Separator block sidebar settings.
The separator block styles.