class GemHadar

def write_gemfile

it doesn't exist. If a custom Gemfile exists, it only displays a warning.
The write_gemfile method creates and writes the default Gemfile content if 
def write_gemfile
  default_gemfile =<<~EOT
    # vim: set filetype=ruby et sw=2 ts=2:
    source 'https://rubygems.org'
    gemspec
  EOT
  current_gemfile = File.exist?('Gemfile') && File.read('Gemfile')
  case current_gemfile
  when false
    secure_write('Gemfile') do |output|
      output.write default_gemfile
    end
  when default_gemfile
    ;;
  else
    warn "INFO: Current Gemfile differs from default Gemfile."
  end
end