gistfile1.rb

require 'bundler/setup'
Bundler.require
require 'google/api_client'
require 'google/api_client/client_secrets'
require 'google/api_client/auth/installed_app'

client = Google::APIClient.new(
  :application_name => 'Example Ruby application',
  :application_version => '1.0.0'
)

key = Google::APIClient::KeyUtils.load_from_pkcs12('./tsundoku.p12', 'notasecret')

client.authorization = Signet::OAuth2::Client.new(
  :token_credential_uri => 'https://accounts.google.com/o/oauth2/token',
  :audience => 'https://accounts.google.com/o/oauth2/token',
  :scope => 'https://www.googleapis.com/auth/gmail.readonly',
  :issuer => my_email,
  :signing_key => key)

client.authorization.fetch_access_token!
                                                                                                                                                           gmail = client.discovered_api('gmail', "v1")

emails = client.execute(                                                                                                                                     api_method: gmail.users.messages.list,
  parameters: {
    userId: "me",
  },
  headers: {'Content-Type' => 'application/json'}
)