jekyll-multiple-languages-plugin cannot be found - ruby

Question

I'm newbie to Ruby and Jekyll. Recently I've tried to install Jekyll Multiple Languages Plugin onto my GitLab Pages instance. I've managed to successfully add
'gem install jekyll-multiple-languages-plugin
bundle install'
into the .gitlab-ci.yml but when I try to add
gems:
jekyll-multiple-languages-plugin
into _config.yml in order to use it on the site my commit fails with the following error:
Using jekyll-watch 1.5.0
Using jekyll 3.4.3
Bundle complete! 3 Gemfile dependencies, 20 gems now installed.
Bundled gems are installed into /usr/local/bundle.
$ jekyll build -d public
Configuration file: /builds/myusername/forty-jekyll-theme/_config.yml
Dependency Error: Yikes! It looks like you don't have jekyll-multiple-languages-plugin or one of its dependencies installed. In order to use Jekyll as currently configured, you'll need to install this gem. The full error message from Ruby is: 'cannot load such file -- jekyll-multiple-languages-plugin' If you run into trouble, you can find helpful resources at https://jekyllrb.com/help/!
jekyll 3.4.3 | Error: jekyll-multiple-languages-plugin
ERROR: Job failed: exit code 1
I have used this method because any attempt to edit Gemfile ends up in commit error. I've also tried to do all presented steps except adding bundle install. In that case I get the same error, but the beginning looks like:
18 gems installed
$ gem install jekyll-multiple-languages-plugin
Successfully installed jekyll-multiple-languages-plugin-1.5.1
1 gem installed
$ jekyll build -d public
I did not manage to find the way to solve it on Stack Overflow nor other sites. For example this did not help

Answer 1

With the awesome support of allejo from Jekyll's IRC channel I've managed to solve the problem. Here are the steps:
I've used Gemfile. Now it looks like
source 'https://rubygems.org'
gem 'jekyll'
group :jekyll_plugins do
gem 'jekyll-multiple-languages-plugin'
end
The second modified thing was .gitlab-ci.yml (the first line - image: ruby - is also included)
image: ruby
pages:
stage: build
script:
# - gem install jekyll
- gem install bundler
# - gem install jekyll-multiple-languages-plugin
- bundle install
- bundle exec jekyll build -d public
# - jekyll build -d public
artifacts:
paths:
- public
only:
- master
It produced a bug, but it was only caused by the lack of declared language which is part of standard plugin configuration.