You must edit your _config.yml file to add Jekyll plugins to your GitHub Pages site.
To add Jekyll plugins to your site, you need to configure your _config.yml file by adding your plugin's Jekyll gem. This is an example of a _config.yml file that has several supported jekyll plugins enabled:
gems: - jekyll-mentions - jemoji - jekyll-redirect-from - jekyll-sitemap - jekyll-feed
If you don't have a _config.yml file, you will need to create one in the root of your GitHub Pages repository. For more information, see Jekyll's official plugins documentation.