This option is only available to users of Code Snippets Pro. 

To begin, open the post, page or custom post type item where you would like the content snippet in the Gutenberg block editor.

Open the block library by clicking on the plus icon or typing / in the editor.

Find the Content Snippet block and drag onto the page. Choose an existing Content Snippet from the menu.

There are several options for controlling exactly how the content snippet will display on your site. You can read more about what these options do here.