The new feature was announced last month but Microsoft says it needs "some additional feedback before making these updates broadly available." That means that if even if you're not an Office 365 subscriber you can download the preview and test it freely until June 30th.
What should I do if I’m already an Outlook for Mac user?
If you are already using Outlook as part of Office 365, this preview isn’t for you. Existing users can get access to the new Google Account features, and all the latest Outlook innovations, simply by joining our Insider Fast community. Just open Outlook, click Help > Check for Updates and then follow these directions.
What if I don’t use Outlook for Mac?
This preview is for you! To try Outlook for Mac with your Google Account and test drive the new updates, visit our preview website and click the Download button. This will install the latest preview version of Outlook 2016 for Mac, which you can use until June 30, 2017. You will receive the same builds and weekly updates as our Insider Fast community. As you use the new features, check out this list of known issues and share your feedback by going to Help > Contact Support in Outlook.
Download the Outlook for Mac Preview from the link below...