This document describes Garelic, an open source tool that allows using Google Analytics for app performance monitoring. It explains how Garelic uses Google Analytics features like User Timing and Custom Variables to track metrics like controller and ActiveRecord response times. Garelic implements this by monkey patching Rails and subscribing to ActiveSupport notifications to capture timing data and report it to Google Analytics. Some limitations are that it can't track redirects, AJAX, or jobs, and page caching breaks reports. Ideas for future work include error tracking and thresholding for slow responses only.