Soon you will be able to access your gmail both online and offline. The guys at Google had started experimenting with it and in fact you can test it yourself by following these steps :
1. Click Settings and click the Labs tab.
2. Select Enable next to Offline Gmail.
3. Click Save Changes.
4. After your browser reloads, you'll see a new "Offline0.1" link in the upper righthand corner of your account, next to your username. Click this link to start the offline set up process and download Gears if you don't already have it.
Good luck and enjoy Gmail offline.