Set up and configure a Project in Google Developer Console

Create a project from here

Enable the Google Sheets API for your project from here

Create a service Account (to get credentials to test with)

(Credentials -> Service Accounts -> "Create Service Account" )

Create a key (Select Service Account -> Keys -> "Add Key" -> "Create New Key" -> "Json")

This should download a .json file. Add this file to your app

Create a google sheet and give the Service Account access

Get the Service Account email from the "Details" tab of the Service Account

Open/create a sheet, hit "Share", then enter the email address of the Service Account.

The following code snippet should get you authenticated and accessing data

scope = 'https://www.googleapis.com/auth/spreadsheets'

file = File.join(Rails.root, 'config', 'googleauth.json')

authorizer = Google::Auth::ServiceAccountCredentials.make_creds(
  json_key_io: File.open(file),
  scope: scope)

authorizer.fetch_access_token!

service = Google::Apis::SheetsV4::SheetsService.new
service.authorization = authorizer

spreadsheet_id = "1ENkiF9XBfmToZAVPPS61TwSQs0tiumjDrmp9_1hOWn0"
range = "Sheet1!A1:B1"

response = service.get_spreadsheet_values spreadsheet_id, range