Based on http://blog.baugues.com/google-calendar-api-oauth2-and-ruby-on-rails. Get week's worth of time entries for a calendar and aggregate them, similar to http://gtimereport.com

calendar_controller.rb

class CalendarController < ApplicationController
  def index
    client = Google::APIClient.new
    client.authorization.access_token = session[:access_token]
    service = client.discovered_api('calendar', 'v3')

    now = Time.now

    start_day = 1
    end_day = now.wday

    if end_day > start_day
      diff = end_day - start_day
    else
      diff = 0
    end

    @result = client.execute(
      :api_method => service.events.list,
      :parameters => {
        :calendarId => 'YOUR_CALENDAR_ID',
        :singleEvents => true,
        :timeMax => now.end_of_day.utc.iso8601,
        :timeMin => (now - diff.days).beginning_of_day.utc.iso8601
      },
      :headers => {'Content-Type' => 'application/json'})

    @grouped_days = Hash.new
    @ungrouped_days = Hash.new
    @totals = Hash.new
    @week_total = 0

    @result.data.items.each do |item|
      day = item.start.dateTime.strftime('%A')
      event_title = item.summary.split(':')[0]
      full_event_title = item.summary
      mins = item.end.dateTime - item.start.dateTime

      if not @grouped_days[day]
        @grouped_days[day] = Hash.new
      end

      if not @ungrouped_days[day]
        @ungrouped_days[day] = Hash.new
      end

      if not @grouped_days[day][event_title]
        @grouped_days[day][event_title] = 0
      end

      if not @ungrouped_days[day][full_event_title]
        @ungrouped_days[day][full_event_title] = 0
      end

      @grouped_days[day][event_title] += mins

      @ungrouped_days[day][full_event_title] += mins

      if not @totals[day]
        @totals[day] = 0
      end

      @totals[day] += mins

      # Track week's hours
      @week_total += mins
    end
  end
end

Gemfile

source 'http://rubygems.org'
gem 'rails', '3.0.3'
gem 'sqlite3-ruby', '1.2.5', :require => 'sqlite3'
gem 'nifty-generators'
gem 'google-api-client', :require => 'google/api_client'
gem 'omniauth', '1.1.0'
gem 'omniauth-google-oauth2', :git => 'https://github.com/zquestz/omniauth-google-oauth2.git'

index.html.erb

<% title "Timesheet" %>

<h2>Total: <%= (@week_total / 3600).to_int %>:<%= "%02d" % ((@week_total % 3600) / 60).to_int %>h</h2>

<h3>Grouped</h3>

<table>
  <thead>
    <tr>
      <th>Day</th>
      <th>Job</th>
      <th>Time</th>
    </tr>
  </thead>
  <tbody>
    <% @grouped_days.each do |day, times| %>
      <tr>
        <td><%= day %></td>
      </tr>
      <% times.each do |job, time| %>
        <tr>
          <td></td>
          <td><%= job %></td>
          <td><%= (time / 3600).to_int %>:<%= "%02d" % ((time % 3600) / 60).to_int %>h</td>
      <% end %>
      <tr>
        <td colspan="2"></td>
        <td><%= (@totals[day] / 3600).to_int %>:<%= ((@totals[day] % 3600) / 60).to_int %>h</td>
      </tr>
    <% end %>
  </tbody>
</table>

<h3>Ungrouped</h3>

<table>
  <thead>
    <tr>
      <th>Day</th>
      <th>Job</th>
      <th>Time</th>
    </tr>
  </thead>
  <tbody>
    <% @ungrouped_days.each do |day, times| %>
      <tr>
        <td><%= day %></td>
      </tr>
      <% times.each do |job, time| %>
        <tr>
          <td></td>
          <td><%= job %></td>
          <td><%= (time / 3600).to_int %>:<%= "%02d" % ((time % 3600) / 60).to_int %>h</td>
      <% end %>
      <tr>
        <td colspan="2"></td>
        <td><%= (@totals[day] / 3600).to_int %>:<%= ((@totals[day] % 3600) / 60).to_int %>h</td>
      </tr>
    <% end %>
  </tbody>
</table>