google_play_verification.rb

class GooglePlayVerification
  require 'google/api_client'

  # Refer: http://jonathanotto.com/blog/google_oauth2_api_quick_tutorial.html
  GOOGLE_KEY    = 'xxxcc.apps.googleusercontent.com'
  GOOGLE_SECRET = 'abcd'
  ACCESS_TOKEN  = 'abcd'
  REFRESH_TOKEN = 'abcd'
  APP_NAME      = 'StockLight'
  APP_VERSION   = '1.0.1'

  def self.process_receipt(transaction_receipt)
    package_name    = transaction_receipt.package_name
    subscription_id = transaction_receipt.product_id
    purchase_token  = transaction_receipt.purchase_token
    self.verify_subscription(package_name, subscription_id, purchase_token)    
  end

  def self.google_api_client
    @@google_client ||= Google::APIClient.new(
      auto_refresh_token:  true, 
      application_name:    APP_NAME, 
      application_version: APP_VERSION
    ).tap do |client|
      client.authorization.client_id     = GOOGLE_KEY
      client.authorization.client_secret = GOOGLE_SECRET
      client.authorization.access_token  = ACCESS_TOKEN
      client.authorization.refresh_token = REFRESH_TOKEN
    end
  end

  # ie. https://developers.google.com/android-publisher/v1/
  def self.verify_subscription(package_name, subscription_id, purchase_token)
    client = self.google_api_client

    # Verify whether the transaction_receipt is valid
    subscriptions = client.discovered_api('androidpublisher', 'v1')
    api_method = subscriptions.purchases.get
    purchases = client.execute(api_method: api_method, parameters: { 
      "packageName"    => package_name,
      "subscriptionId" => subscription_id,
      "token"          => purchase_token
    }).data
  end

end

Hi! Thank you very much for this gist, especially for explenation how to get credentials.
But, in the Sample usage section you are showing that: subscription = GooglePlaySubscriptionVerification.new(package_name, product_id, purchase_token) will result in Google::Apis::AndroidpublisherV2::SubscriptionPurchase object, but actually you will get a GooglePlaySubscriptionVerification object. To get Google::Apis::AndroidpublisherV2::SubscriptionPurchase you will need to add .verify to GooglePlaySubscriptionVerification.new(package_name, product_id, purchase_token).

To verify a product purchase:

def verify_product
  android_publisher.get_purchase_product @package_name, @product_id, @purchase_token
end

Response looks like this:

Success - #<Google::Apis::AndroidpublisherV3::ProductPurchase:0x00000000000
 @consumption_state=0,
 @developer_payload="",
 @kind="androidpublisher#productPurchase",
 @order_id="<order_id>",
 @purchase_state=0,
 @purchase_time_millis=1582251976970,
 @purchase_type=0>