A quick note for anything trying to get Gears to work with Firefox 3.5 and 64-bit Linux: follow the steps in this post. The post is for gcc 4.3.3, but it worked just fine with my gcc 4.4.1. Beware: the svn checkout step will take many many minutes.
sudo apt-get install build-essential subversion m4
svn checkout http://gears.googlecode.com/svn/trunk gears-trunk
patch -p0 -i gears-gcc433.diff
chmod +x third_party/gecko_1.9/linux/gecko_sdk/bin/xpidl
The installable file will end up in
./bin-dbg/installers/ (that is, something like
/home/foo/gears-trunk/gears/bin-dbg/installers), with a name like
gears-linux-x86_64-dbg-0.5.34.0.xpi. In Firefox, pick “Open File” from the File menu and browse to that directory; pick the XPI file and you should be good to go. I’ve only tested it with offline gmail, but that seems to work fine.