Here is how you can use Gmail Offline:

Open your Gmail account on a computer and access the settings using on the top right corner. On the pop-up menu, selectSee all settings

The click on the “Offline” tab from the top navigation bar.

On the next page you will have to check the box reading “Enable offline mail”. This will turn on offline mode for Gmail.

Once the mode is done, click on “Save Changes” button at the bottom.