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.
Gutenberg is a soon-to-arrive project in WordPress Core that is best understood as a layout builder: a way of more effectively including and arranging various types of post content.
Gutenberg's short-term effects on a WordPress developer's day-to-day work will be relatively minor. This is partly because WordPress already has at least one high-quality layout builder that provides many of the advantages Gutenberg hopes to offer in the short term.
In the longer term, Gutenberg will hopefully help to consolidate and improve the WordPress ecosystem, with mixed effects on existing theme and plugin developers, but positive effects for agencies, end users, and WordPress as a whole.