Taken from https://github.com/unixcharles/google_calendar_api_v2 and modified for OAuth2

All you need to survive a syncronization project with Google Calendar using GData 3.0 + OAuth 2.0

Usage

Create an application.rb file and put the following in it.

require 'sinatra'
require 'rubygems'
require 'google_calendar_oauth2'

client = GoogleCalendar::Client.new "google_client_id", "google_client_secret", "http://localhost:4567/oauth2callback"

before do
  unless GoogleCalendar.connection.authorization.access_token || request.path_info =~ /^\/oauth2/
    redirect client.redirect_to
  end 
end

get '/oauth2callback' do
  GoogleCalendar.connection.authorization.code = params['code']
  GoogleCalendar.connection.authorization.fetch_access_token!
  redirect '/create_event'
end

Create your Gemfile with the following.

source 'http://rubygems.org'

gem 'sinatra'
gem 'google_calendar_oauth2'

Finally, run

bundle exec ruby application.rb

In your application directory and go to http://localhost:4567/

#Calendar

Find a Calendar

calendar = GoogleCalendar::Calendar.find_by_name('Calendar Name')

List your Calendars

calendars = GoogleCalendar::Calendar.list

Create a Calendar

calendar = GoogleCalendar::Calendar.create({'summary' => 'New Calendar', 'timeZone' => 'America/Chicago'})

#Event Find an event

GoogleCalendar::Event.find_by_name(calendar.id, 'some event name')

List events for a calendar

events = GoogleCalendar::Event.list(calendar.id)

Create an event

GoogleCalendar::Event.create({'summary' => 'Some cool event everyone wants to go to.', 'start' => {'dateTime' => '2012-01-29T12:00:00-06:00'}, 'end' => 'dateTime' => '2012-01-29T13:00:00-06:00'})