Finally Google offered the offline feature for Gmail via Gears, Google took so much time to offer offline Gmail although Gears is their own project. As usual this feature is not yet available to all users, it will be rolled out in phases.
How to set Gmail for offline?
This feature is rolled out slowly so wait until you get this one for your account, you can check for this in Gmail Labs tab (Settings > Labs), if you see Offline Gmail enable it, save changes and click the link offline link that will be displayed next to your gmail username. Both Google Reader and Google Docs already support this feature.
What you can do with offline Gmail?
You can read messages, star and label them, and do all of the things you’re used to doing while reading your webmail online. Any messages you send while offline will be placed in your outbox and automatically sent the next time Gmail detects a connection. And if you’re on an unreliable or slow connection (like when you’re “borrowing” your neighbor’s wireless), you can choose to use “flaky connection mode,” which is somewhere in between: it uses the local cache as if you were disconnected, but still synchronizes your mail with the server in the background. Our goal is to provide nearly the same browser-based Gmail experience whether you’re using the data cached on your computer or talking directly to the server.