Pre-S: For more Gutenberg, you can watch our Up and Running Gutenberg Roundtable discussion, where we discuss what's happening with Gutenberg in the next year. Watch the video (or read the transcript) here.
As it will exist in WordPress 5.0, Gutenberg is mainly a replacement for the WordPress Visual editor. "Programming in Gutenberg" is confined to the "Edit Post" Screen: creating Blocks--which are the layout (and post metadata) elements that Gutenberg uses--and the metabox interfaces we find there.
Gutenberg is likely to replace or change several additional systems over time, starting with widgets, and possibly later impacting custom page templates and menus.
Most technical systems in WordPress—backbone functionality like working with action and filter hooks, querying the database with WP_Query, and enqueueing scripts and styles—will not change at all as a result of Gutenberg.