Installing a gem is not sufficient to make it available in a Rails project. You need to add it to the Gemfile so that you can manage the dependencies with Bundler.
Edit the 'Gemfile' and add the gem
gem 'numbers_and_words'
Then run bundle again to update the Gemfile.lock
$ bundler
This will make the gem available to the app. It will also autorequire the gem on boot, if the gem uses the standard naming conventions.
It seems this gem is name correctly. Otherwise, you can explicitly require it by setting a require
option in the gemfile
gem 'numbers_and_words', require: 'numbers_and_words'
If you just installed the gem locally and you didn't configure the Gemfile, the application will crash once you will deploy it.