APIに必要なもの($ANDROID_CLIENT_IDとかに代入する)を揃えたら、実行すればいけるはず。

# アンドロイドのレシートチェック
def check_receipt_for_android
  def fetch_receipt_for_android(product_id, purchase_token, token)
    require 'net/http'
    require 'net/https'
    require 'uri'
    require 'json'

    # path = "https://www.googleapis.com/androidpublisher/v2/applications/#{$ANDROID_PACKAGE_NAME}/purchases/products/#{product_id}/tokens/#{token}"
    path = "https://www.googleapis.com/androidpublisher/v1.1/applications/#{$ANDROID_PACKAGE_NAME}/inapp/#{product_id}/purchases/#{purchase_token}?access_token=#{token}"

    uri = URI.parse path
    https = Net::HTTP.new(uri.host,uri.port)
    https.use_ssl = true
    https.verify_mode = OpenSSL::SSL::VERIFY_NONE

    headers = {}
    headers["accept"] = "application/json"
    headers["content-type"] = "application/json"

    # 参考: https://stackoverflow.com/questions/7346398/nethttp-failure-to-access-google-docs-list-data-api
    https.start do |http|
      return http.get(uri.request_uri, headers).body
    end
  end

  # main
  # gemfile => gem 'google-api-client', '~> 0.8.6'
  require 'google/api_client'

  client = Google::APIClient.new(
    application_name:    '適宜',
    application_version: '1.2.3',
  )

  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.eisbahn.jp/yoichiro/2011/10/oauth2-0_google-api.html
    client_id:     $ANDROID_CLIENT_ID,
    client_secret: $ANDROID_CLIENT_SECRET,
    refresh_token: $ANDROID_REFRESH_KEY,
  )

  p client.authorization.access_token #=> nil

  client.authorization.refresh!

  product_id = "product_id"
  purchase_token = "xxxxxxxxxxxx.xx-xxxx-kRhDk04x2ahVCKPxCDYAnxuxxxW1FX5kn8rEZf66d7BpItG411lyUr-xxxxxxxxxxxxxnBaDPZakoUBhUliHrbzwOT1ZoLvP-5uy7Fxii3DsxxxxxxMHf-n_N3c-ozSgxxxJ-LUXxx-bxxxf1Tw"
  receipt = get_android_refresh_token(product_id, purchase_token, client.authorization.access_token)

  pp JSON.parse(receipt, {symbolize_names: true})
end