Google first announced Gmail offline in August of last year, and when they said it was "only the beginning of our HTML5-powered offline journey", well, they were right. Google have said they've been listening to everybody's feedback and have now pushed some updates to the Chrome Web Store app.
The first update is the addition of a 'settings' page. From here, you can choose how many days of mail you'd like to synchronize, 7, 14 or 31 days. This means you can sync a bunch of e-mail before you get on the plane, or a bus/train and sort through 31 days of e-mail, all while you're offline.
Google also baked in a few more updates to Gmail offline:
Improved attachments: All attachments are now downloaded and available for offline use
Keyboard shortcuts support: If you have keyboard shortcuts enabled in Gmail, your setting will transfer over to the Gmail offline app. If you're not sure what you can do with keyboard shortcuts, try pressing '?' next time you're using Gmail or Gmail offline.
Numerous performance enhancements: Messages and attachments now download at a faster rate and some bugs have been fixed.
You can install the Gmail offline app from the Chrome Web Store.