I am using Geva Kozacs' patch & builds for firefox-3.5 (yep, 3.5!), which work perfectly with Gmail (+ Offline addon from labs) and Google reader.
Links: http://gkovacs.xvm.mit.edu/google-gears-firefox-3.5-x86_64/ http://blog.celogeek.fr/linux/linux-trucs-et-astuces/google-gears-compilation/ So what are the developers waiting for? Make an official "alpha"/"beta" release already, there are already a handful of people using unofficial patches and it's getting tiresome! I don't believe there's need to make a test suite, since the Linux users are usually pedantic and report errors back to upstream. Cheers! :-)
