Most notably, Gmail is getting a unified inbox option witch will allow you to view all of your inboxes in one place – even for Yahoo and Outlook accounts. If you’d like to access the new feature, you’ll need to slide out the navigation drawer and select the “All Inboxes” option. Additionally, the new update brings the popular Conversation View feature to all of your connected Yahoo, Outlook and other IMAP/POP accounts.
Google didn't clarify when the update would begin rolling out, so it may be a few days before it makes its way to your device.
[Source: Android Authority]