Where is my gem file located so that i can rake test

Googling around, I found that apparently adding to the gem file will
fix my problems, but where do i find the gem file? what is the name of
the gem file?

"group :test do
  # Pretty printed test output
  gem 'minitest' ####### add this line #######
end"

Here is what happens when i run rake test, please note that i have
already installed minitest (2.7.0)

It is called Gemfile and should be in the top level of the rails
project (in the directory with app, config and so on.

Colin

Thank you Colin. Issue resolved.