In Event Builder, navigate to the event page you want to add Google Analytics to and click the Advanced tab in your toolbar. Scroll down to the Additional Footer Content field. This is where you'll paste your Google Analytics script. Be sure to save your changes!
You CAN add it to the header if desired. We recommend adding it to the footer because this causes it to load last, which means it won't affect the speed of rendering everything on the page. It's better from a performance standpoint, but overall either option is fine.