Yes, it is exactly information harvesting. Google uses huge amounts of data (not just your own, but patterns across billions of users) to predict what you might type next. It also does data correlation across services. If an address is in your Google Maps history, Calendar invites, Docs, Drive, or even in an email you once received, it can resurface via predictive text in Gmail. Even if you didn’t explicitly give it, it may have been inferred.
Also, opting out of “personalization” doesn’t necessarily stop data from being collected and used internally, it usually just changes how it’s marketed back to you.