This is a core block and thus available by default in Gutenberg WordPress editor. No download required.
WordPress Custom HTML Block in Gutenberg allows you to write your own Custom Code and preview it too. This block can be considered as the replacement of the HTML view you had in the classic WordPress editor. You can write whatever valid HTML code inside and it will output the result on the frontend (not display the code like the code block).
Custom HTML Block Features
A really simple HTML block to write custom HTML in Gutenberg. In the toolbar, it just has options to preview and HTML (to write code). There are no options in the sidebar settings panel for this block.
The output on the frontend for the custom HTML block.
This is how it looks in the Gutenberg editor by default.
There are currently no additional settings for custom HTML block in the sidebar.