Standard

For local development and use outside of GH Pages, it is sufficient to add a plugin to the Gemfile under the :jekyll_plugins group to enable it.

group :jekyll_plugins do
  gem "jekyll-feed"
  gem "jekyll-sitemap", "~> 1.4"
  gem "jekyll-seo-tag", "~> 2.7.1"
end

GitHub Pages

But when it comes to hosting on GitHub Pages, you must also add those to the plugins field in _config.yml.

However, for a plugin is already used by the theme then you don’t have to add that to plugins. Like the seo tag is used in the Minima head.html file and installed as jekyll-seo-tag within the theme.

I found was all true when using minima as a theme for my jekyll-blog-demo site.

Alternate

When I used my fork of minima, I found that I did not have to add to plugins field. Perhaps because I installed the theme with the Remote Theme plugin

See for example: