Gmail Checker version 2.5.0.90 which supports 2-step verification is now available. You can get it here (51 KB).

Important: The extension does not work with application-specific passwords. You must use your normal password and then enter the SMS verification code when prompted.

A few people aren't happy with the icons which I found quite okay. Just for experiment, this release uses icons created by RoksVempire. Feel free to send me your own icons if you think that they are better.

Note: The icons must be in both 24x24 and 16x16 size.

Update 6/4:

The fix for 2-step verification doesn't work any more. Google has changed the way Gmail Atom feeds are authenticated. I'm working on the solution.