Gmail has updated their UI to new Material Design. After material design introduced by the Google they can change the all the product UI to material design one by one. Lastly, Google changes the UI for Gmail now.
If you log in to your Gmail account it will show the notification in the top right corner of the Gmail screen below the setting option. You want to try the Gmail new User Interface. You can click the try button it will take to the new design of the Gmail.
It will not show the notification you can change the Gmail new design manually by clicking the setting gear icon present at the top right corner of the Gmail Screen. after the click, it will show drop-down menu in that you can see the try new option present at first.
Just click that now you can take Gmail new appearance new look.
A lot of changes done in the Gmail new design.
If you are not interested in the new UI you can go back to old one by clicking the same setting gear icon it will show the option at the same location. Go back to classic Gmail. Just click it it will take to old design.