Google announced last January that a new integrated view for Gmail was coming and while it had been delayed, starting this month, users who have not opted-in will begin seeing the new experience by default, but can revert to classic Gmail via Quick settings (gear in top right). Within the next two months, this will become the default experience with no option to revert back.
See this support article for more background – https://support.google.com/mail/answer/11555490
