In the google sync settings window it states that two of my three custom fields will get populated. However, this is not happening. I am wondering how I should formulate the event subject - when I add a new event in my google calendar - for the auto population to happen correctly? 

Hi autopopulate fields is based on same email. For example you have customer with email X and if you enter that email as attendee email in Google Calendar, EA will check if same email was  used for some previous appointments. If that is the case other fields will be populate based on that previous.
Best regards, Nikola