For now, it would work with Internet Explorer on Windows Mobile 5 and 6.
A company developer wrote about this app in a blog post:
The mission of Gears is to extend the capabilities of web browsers. It is clear to us that mobile browsers can benefit just as much as desktop ones. By adding features to mobile browsers, it becomes possible to deploy an increasing number of mobile applications as web apps.
They are keeping the Gears API consistent across all platforms so as to make it easier for the developers to code their applications for several platforms.