Yeah you heard it right, Gmail is now not just great in terms of features but also Google has introduced their usual white and wider interface in Gmail as well. So all the people out here who loved and wished Gmail to be a reflection of Google can now actually feel the essence in Gmail’s new interface.
To see and apply the neat and clean interface shown below in the images all you need to do is to just select any out of “Preview” and “Preview (Dense)” themes in the Themes tab in Gmail Settings. Here’s what one of the new themes currently looks like: And in conversation view: 
Click the images above to see larger versions. Note: Some Labs features may look a little strange in the new themes. However Gmail Team is working to fix these issues and will roll out changes in the coming months. Gmail will also roll out some updated themes that embody the same design principles but are better suited to working in a dark environment, use a different color palette, or include the illustrations that many of us love to see around our inbox(including me) 
.
Look out for these and other new features over the next few months. In the meantime, try out the new themes as a preview of the future of Gmail. Tell us what do you think new Gmail interface. 
Source: Official Gmail Blog
https://www.electronicgurudev.in/technology/check-out-gmails-new-look/