Offline Gmail has seen several recent feature additions like offline attachments and "Flaky Connection Mode" that serve to make it a true "offline" application. It is possible to read and reply to messages without an active Internet connection, similar to standalone email programs. Existing users of Offline Gmail don't have to make any changes, Google will enable it by default. Anyone else who wishes to use the feature can now find it inside Gmail proper.
Future offline web-apps from Google may not be based on Gears, HTML5 becoming favored by developers. Google's efforts toward HTML5 standardization will give them an advantage in developing offline apps -- though, hopefully they will work to provide migration paths for Gears-based software.