Gem strong parameters not installed

On the official gem page strong_parameters on GitHub says the following:

  

This plugin is only fully compatible with Rails versions 3.0, 3.1 and 3.2 but not 4.0+, as it is part of Rails Core in 4.0. An unofficial Rails 2 version is strong_parameters_rails2.

What translated means:

  

This plugin is only fully compatible with versions 3.0, 3.1 and 3.2 of Rails, but not with versions from 4.0, since version 4.0 this gem has already been integrated into your core . An unofficial version for Rails 2 is strong_parameters_rails2.

That is, since you are using version 4.1, this gem is already installed, even if it does not appear in Gemfile.