This is a core block and thus available by default in Gutenberg WordPress editor. No download required.
The WordPress Classic Block in Gutenberg brings back the old Classic Text Editor you are familiar with. So generally you do not have to disable Gutenberg. You can still use Gutenberg with Classic Block which only to write your complete post or page.
Classic Block Features
The classic block is basically the same TinyMCE editor you have been using in WordPress for years. Thus it has all the same controls and options you had in the old WordPress editor. The added benefit here is that it is a block and can be added multiple times on a page or post where ever you need it.
The output on the frontend of the classic block is same as the old WordPress editor.
This is how it looks in the Gutenberg editor. You must recognize it easily as it is the same old TinyMCE editor.
The classic block has no settings in the sidebar panel as of now.