Offline Gmail is built into Gmail so new users can work even when lost internet connection easily.
On the new Gmail Google has added a nice feature or offline Gmail allows users to check the email sent/received without the internet easily.
To use this feature you must first make sure that you are using the latest version of Gmail, if you still use the old interface, just select the gear icon in the top right corner of Gmail and select Try Gmail. Then open the settings:
Or you can go directly to it: https://mail.google.com/mail/u/0/#settings/offline.
And select Enable offline:
By default, Gmail will use 8GB on your hard drive to back up your sent/received emails. If you want to download attachments, select Download Attachments. There are also two options for retaining data when you sign out of your Google Account and clear your data when you sign out of Google. Once you have set up just select Save changes then all settings will be saved and email will be synchronized to the machine.
This is a good feature for people who regularly work on Gmail, or to review emails exchanged with partners, customers … and attachments in the email.