Plugin developers will be able to completely integrate it in every part of WordPress including posts, pages, custom post types, and sidebars without hacking TinyMCE or squeeze their entire feature behind a toolbar button. Today, every plugin that extends WordPress does it in a different way; Gutenberg’s blocks provide a single, easy-to-learn entry point for an incredible variety of extensions. Some developers already started to port there plugin over, and found that they’re easier to build and have much improved UI.
Widgets and short codes will continue to work as it’s working prior to Gutenberg. The Gutenberg editor seeks to offer new ways to interact with those existing elements on the site. By creating editable blocks for these elements, users will have more control over the visual display of their content without having to know any code.
Template is going to work in same manner. In the Gutenberg editor you can select the theme template you wish to use for layout and design on the right-hand side.
For now Gutenbergy will not replace Page Builders/editors like WP Page Bakery, Elementor, and others. As Gutenberg continues to evolve, it’s possible it may replace some of the features and functionalities offered by these Page Builders. We believe that by the Page Builders will evolve and there maybe advanced functionalities may be offered which Gutenberg will lack.