Use github-pages gem only when building via GitHub Actions

Allows inclusion of gems that will be relevant in
other CIs such as GitLab.

See pull request carpentries#521 for additional context

Gemfile

@@ -7,8 +7,45 @@ git_source(:github) { |repo_name| "https://github.com/#{repo_name}" }
 # Synchronize with https://pages.github.com/versions
 ruby '>=2.7.1'
 
-gem 'github-pages', group: :jekyll_plugins
+group :jekyll_plugins do
+  if ENV["GITHUB_ACTIONS"]
+    # When building GitHub Pages via GitHub Actions
+    gem 'github-pages'
+  else
+    # Or elsewhere. Dependency list from 'github-pages'
+    # See: https://github.com/github/pages-gem/blob/75fd58be0f294a6bf55a1990643838d5984a1f62/lib/github-pages/dependencies.rb#L8
+    gem 'jekyll', '>= 3.9.0'
+    gem 'jekyll-sass-converter', '>= 1.5.2'
+    # Converters
+    gem 'kramdown', '>= 2.3.1'
+    gem 'kramdown-parser-gfm', '>= 1.1.0'
+    gem 'jekyll-commonmark-ghpages', '>= 0.1.6'
+    # Misc
+    gem 'liquid', '>= 4.0.3'
+    gem 'rouge', '>= 3.26.0'
+    # gem 'github-pages-health-check', '>= 1.17.1'  # Not necessary outside GitHub
+    # Plugins
+    gem 'jekyll-redirect-from', '>= 0.16.0'
+    gem 'jekyll-sitemap', '>= 1.4.0'
+    gem 'jekyll-feed', '>= 0.15.1'
+    gem 'jekyll-gist', '>= 1.5.0'
+    gem 'jekyll-paginate', '>= 1.1.0'
+    gem 'jekyll-coffeescript', '>= 1.1.1'
+    gem 'jekyll-seo-tag', '>= 2.7.1'
+    # gem 'jekyll-github-metadata', '>= 2.13.0'  # Not necessary outside GitHub
+    gem 'jekyll-avatar', '>= 0.7.0'
+    gem 'jekyll-remote-theme', '>= 0.4.3'
+    # Plugins to match GitHub.com Markdown
+    gem 'jemoji', '>= 0.12.0'
+    gem 'jekyll-mentions', '>= 1.6.0'
+    gem 'jekyll-relative-links', '>= 0.6.1'
+    gem 'jekyll-optional-front-matter', '>= 0.3.2'
+    gem 'jekyll-readme-index', '>= 0.3.0'
+    gem 'jekyll-default-layout', '>= 0.1.4'
+    gem 'jekyll-titles-from-headings', '>= 0.5.3'
+  end
+end
 
 if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.0.0')
     gem 'webrick', '>= 1.6.1'
-end
+end