here (http://code.google.com/p/gears/issues/detail?id=335#c33) I found the gears-linux-x86_64-opt-0.5.33.0.xpi
or 0.5.25

me: karmic ubuntu 9.10 , x64, firefox 3.5.5
seems working !

here (http://gears.google.com/) it says:
System requirements:
* Windows XP/Vista
* Firefox 1.5+ and Internet Explorer 6.0+

* Linux (details)
* Firefox 1.5+
* 32-bit OS (64-bit not supported)

after my version install says:
Gears is installed
Current version:
0.5.33.0;developer;opt;linux;firefox