One of the great new features of the new Gmail is the ability to work offline without the old ugly Chrome plugin. I can tell you that is one of the biggest issues for all my clients, I am glad it is solved.
Enable it by opening your G Suite Admin panel then Apps > G Suite > Gmail > User Settings. Select the options “Allow my users access to the new Gmail UI and features” then “Enable Gmail web offline”, as the screenshot below:
Now all the users which opt into the new interface will be able to read the last 90 days of email offline.