Microsoft seems to be testing a new feature that lets users add Google services to their Outlook accounts, as revealed by Twitter user Florian B and reported by The Verge. If available on your account, you should see a pop-up that says, "Add your Google Mail and Calendar to Outlook and easily manage everything in one place." Doing so allows you to switch between your Outlook and Gmail inboxes (though this causes the entire browser tab to refresh) and browse your Drive files. It's not clear how Google Calendar is integrated into Outlook.
At the moment, you can only select a single Gmail account to sync with Outlook, which could be frustrating since so many people use multiple Gmail addresses. This isn't the first time the two competitors have joined forces. In 2017, Microsoft added Google Calendar and Contacts functions to its desktop-based version of Outlook 2016. As testing continues, more Outlook users could have the feature applied to their accounts. We've reached out to Microsoft for clarification and will update this post if we hear back.
Update: Microsoft has confirmed the test, with a spokesperson telling Engadget "We are always looking for new ways to extend the best email experience to our customers and can confirm that we're experimenting with a small set of Outlook.com users to learn and gather feedback."