It replaced my theme with an awful theme and prevented me from logging in to the admin to disable and delete it.
That sounds like a really bad experience, again sorry you had to go through that. As Gutenberg is just the editor and doesn’t come with a theme, I do wonder if something else happened? What theme were you using before? I would love to get to the bottom of the issue you experienced as that may help others. Thank you for any extra insight you can give.