Benoît de Boursetty on the official Gmail blog:
Gmail Offline is a Chrome Web Store app that’s intended for situations when you need to read, respond to, organize and archive email without an internet connection. This HTML5-powered app is based on the Gmail web app for tablets, which was built to function with or without web access. After you install the Gmail Offline app from the Chrome Web Store, you can continue using Gmail when you lose your connection by clicking the Gmail Offline icon on Chrome’s “new tab†page.