Microsoft seems to be testing a new integration in Outlook.com which will bring Google services into the experience. The capability was first spotted by Twitter user Florian B, who pointed out that not only is it possible to add a Gmail e-mail inbox, but you can also attach files from Google Drive into e-mail messages, as well as access your Google Calendar on Outlook.com.
As it stands, the feature seems to be very limited in terms of availability, and it doesn't seem to be available for us just yet. This might be because it's still in early testing, as Florian points out that switching between Outlook and Gmail accounts refreshes the entire page, and you also can't add more than one Google account (via The Verge).
Otherwise, the integration seems to be much like what you find in the Outlook apps, which already let you manage accounts from a multitude of services. The Mail and Calendar apps on Windows 10 can also integrate e-mail and calendar from Google, but you can't attach Google Drive files as easily, so this could be an improvement for desktop users.
It's currently unknown when the feature will be more widely available, but we've reached out to Microsoft for more information, and we'll update this article if there's anything new.