Following the instructions that I found on the Google Gears Google Group, I repackaged Google Gears as a Flock extension. (The only other converted Gears extension that I found seems to be for OS X.)

You can install the extension by clicking here.

UPDATE (24 April 2008): Updated the extension to use the latest version of Gears (0.3.14.0). Get it here.

UPDATE (18 May 2008): If you need an older version of Gears, you can get it here. (For example, the current version of Gears for Mac only works on 10.4 or later. I run 10.3.9, so I had to download Gears 0.1.56.0.)