Could not find gem 'github-pages' in locally installed gems

Question

After I execute the bundle exec jekyll serve I get the following error

Could not find gem 'github-pages' in locally installed gems.
Run bundle install to install missing gems.

What am I doing wrong in the Gemfile ?

source "https://rubygems.org"

gem "github-pages", group: :jekyll_plugins

gem "tzinfo-data"
gem "wdm", "~> 0.1.0" if Gem.win_platform?

# If you have any plugins, put them here!
group :jekyll_plugins do
  gem "jekyll-paginate"
  gem "jekyll-sitemap"
  gem "jekyll-gist"
  gem "jekyll-feed"
  gem "jemoji"
  gem "jekyll-include-cache"
  gem "jekyll-algolia"
  gem "minimal-mistakes-jekyll"
end