You will need a Google or Gmail Account. If you don't have one, get one here (it's free).
Go to http://www.google.com/analytics/ and log in with your Google or Gmail account. Click Admin in top menu, and select Create new account from the Accounts drop down menu.
In the Website URL input field, enter your GivenGain charity URL, e.g. "https://www.givengain.com/c/abccharity/" (you'll be able to view the URL under "Settings > Account" in the "Charity URL (optional)" field). Continue, and copy the code that appears on the next page.
In your GivenGain charity account, visit "Settings > Analytics" and paste the Google Analytics code into the "Tracking Code" field and click "Start Tracking". Go back to the Google Analytics window and click the "Finish" button.
That's it! Your GivenGain URL will now be added to your Google Analytics "Website Profiles". It can take Google up to 24 hours to start displaying your traffic reports, and your status will display as "Tracking code not verified" until it has started processing your data.