Chrome 12, removes the built-in Gears plugin, which's still being used in Gmail. Google no longer maintains Gears, and currently focuses on implementing offline support using HTML5.
Google says that you'll only need to wait for a few weeks for HTML5 version of the feature or you can still older versions of Firefox, Internet Explorer and mail client such as Thunderbird or Outlook if in cased you need to use Gears.
It's not wise decision, if the HTML5 offline Gmail wasn't ready for release, removing Gears from Chrome could have been delayed.
"The new Gmail Offline capability is targeted for delivery as a Chrome browser web app this summer. As we move the Gmail Offline capability to a Chrome web app, we will deprecate the Google Gears-based Gmail Offline. This coincides with the version 12 release of the Google Chrome browser which no longer supports Gears. As a result, Google Gears-based Gmail Offline will no longer work with the Chrome browser as of Tuesday May 24, 2011. Google Gears-based Gmail Offline will continue to work in Internet Explorer 8 and Mozilla Firefox 3.6," explains Google.