google_play_verification.rb

# This class shows uses version 0.28.7 of the ruby google-api-client gem circa April 2019
# to query the Google Play subscription API.
#
# If using an older version of the google-api-client gem (ie. version 0.8.x), instead refer to:
# https://gist.github.com/jkotchoff/e60fdf048ec443272045/e3e2c867633900d9d6f53de2de13aa0a0a16bb03
#
# Sample usage:
#
#   package_name = 'com.stocklight.stocklightapp'
#   product_id = 'com.stocklight.stocklight.standardsubscription'
#   purchase_token = 'kmigoi....4YuSQtU8U'
#   subscription = GooglePlaySubscriptionVerification.new(package_name, product_id, purchase_token)
#
#   => #<Google::Apis::AndroidpublisherV2::SubscriptionPurchase:0x007fbcb3b89698 @auto_renewing=false,
#         @cancel_reason=0, @country_code="AU", @developer_payload="", @expiry_time_millis="1468673994725",
#         @kind="androidpublisher#subscriptionPurchase", @price_amount_micros="9490000", @price_currency_code="AUD",
#         @start_time_millis="1458133289294">
#
#   subscription_cancelled = subscription.cancel_reason.present?
#
# https://developers.google.com/android-publisher/api-ref/purchases/subscriptions
class GooglePlaySubscriptionVerification
  require 'google/apis/androidpublisher_v3'
  require 'googleauth'
  require 'googleauth/stores/file_token_store'

  # These credentials come from creating an OAuth Web Application client ID
  # in the Google developer console
  #
  # refer: https://www.youtube.com/watch?v=hfWe1gPCnzc
  #
  # ie.
  # > visit http://console.developers.google.com
  # > API Manager
  # > Credentials
  # > Create Credentials (OAuth client ID)
  # > Application type: Web Application
  # > Authorised redirect URIs: https://developers.google.com/oauthplayground
  # * the resultant client ID / client secret goes in the following GOOGLE_KEY / GOOGLE_SECRET variables
  # > visit: https://developers.google.com/oauthplayground/
  # > Click the settings icon to show the OAuth 2.0 configuration
  # > Tick 'Use your own OAuth credentials'
  # > Enter the OAuth Client ID and OAuth Client secret that you have just created
  # > Check the entry for 'Google Play Developer API v2' in the scopes field and click 'Authorize APIs'
  # > Click 'Allow'
  # > Click 'Exchange authorization code for tokens'
  # * the resultant Refresh token and Access token go in the following REFRESH_TOKEN / ACCESS_TOKEN variables
  GOOGLE_KEY    = 'xxxcc.apps.googleusercontent.com'
  GOOGLE_SECRET = 'abcd'
  ACCESS_TOKEN  = 'abcd'
  REFRESH_TOKEN = 'abcd'

  Androidpublisher = Google::Apis::AndroidpublisherV3

  # @param package_name - refers to the Google Play package name for the app
  # eg. 'com.stocklight.stocklightapp'
  #
  # @param product_id - refers to the id of the subscription type being checked
  # eg. 'com.stocklight.stocklight.standardsubscription'
  #
  # @param purchase_token - the purchase token receipt
  # eg. 'kmigoi....4YuSQtU8U'
  def initialize(package_name, product_id, purchase_token)
    @package_name = package_name
    @product_id = product_id
    @purchase_token = purchase_token
  end

  # Throws Google::Apis::ClientError if an invalid parameter is provided
  def verify
    android_publisher.get_purchase_subscription @package_name, @product_id, @purchase_token
  end

  private

  def android_publisher
    android_publisher = Androidpublisher::AndroidPublisherService.new.tap do |publisher|
      publisher.authorization = client
      publisher.authorization.fetch_access_token!
    end
  end

  def client(scopes = [Androidpublisher::AUTH_ANDROIDPUBLISHER])
    Signet::OAuth2::Client.new(
      authorization_uri: 'https://accounts.google.com/o/oauth2/auth',
      token_credential_uri: 'https://accounts.google.com/o/oauth2/token',
      client_id: GOOGLE_KEY,
      client_secret: GOOGLE_SECRET,
      access_token: ACCESS_TOKEN,
      refresh_token: REFRESH_TOKEN,
      scope: scopes,
    )
  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>