This starter kit is not maintained. I'd suggest instead checking out this article, or if you want an example of a github pages jeykll repo using bootstrap 4 (not using a gem as GH pages doesn't support that) check out this repository.
To use: when you have the dependancies as per
the docs you should run
bundle install.
HTML files are based on Minima - if you need to configure the post or page layouts, copy those files in as needed.
Bootstrap variables can be overriden in _sass/custom.scss - refer
to the full list of available values.
Note that GitHub Pages doesn't run gems that aren't on their predefined list, so it won't work with their autocompile, you'll need to push up and serve the compiled files.