Gutenberg is not yet merged into core, and as far as I can tell there is no hard timetable on when this will happen. You can keep an eye on progress by looking at the milestones on GitHub or Make WordPress Core.
The original plan seems to have been for it to be merged in early this year, but that was never realistic. For now the FAQ just says “2018”.
If you want to test it, or help develop it, you should do that now with the plugin, not wait until it is merged.
@jakept, thank you!
I just wondered if there’s any difference in using the plugin and getting Gutenberg from core itself.
I maintain about 200 WordPress sites and am looking for a reliable way to test the compatibility with Gutenberg.
So I will do this via the plugin.