Just remove the _config.yml file from repo and it will work.
When you added _config.yml file then GitHub pages think you are using a website with jekyll. Now you have to add a index.markdown or just index.md to show your home page. If you add a index.markdown file it will be work.

Now you have to read offical docs to use jekyll https://jekyllrb.com/ .

Pages broken before I add _config.yml, on previous commit site work, but after modify README.md in gitbook I have 404 error. In _config.yml set theme for md-files

I’m revert GitBook commit, but error don’t fix.

I rollback to working commit and site will work. Where I have problem with GitBook commit?

I guess you have problem with config. If you are using jekyll please refer to docs.

Config generated automatic by GitLab and correct. I don’t use Jekyll manual, I only add Markdown files, Jekyll themes applied from repository setting. Now all work fine, I rollback and remove old branch, remove about page info and draft in GitBook.