Opening Gmail with an old version Gears enabled resulted in a blank white window. Disabling it brought Gmail back but, obviously, without the offline features. An updated version of Gears has been available for a while now, but the process of updating isn’t really obvious and just re-installing/updating Gears didn’t work for me for some reason.
You have to open the terminal and run the uninstall command for the old version of Gears before I was able to install the current version, which is 0.5.15.0 at the moment:
$ /Library/Internet\ Plug-Ins/Gears.plugin/Contents/Resources/uninstall.command
removing installer package receipts
Gears for Safari was uninstalled succesfully.
After this is done you can restart Safari, go to Gmail (or any other Gears aware site) and enable the offline mode again. During this process you’ll be prompted to install the current version of Gears. Offline happiness can continue from now on.