I went to some pretty careful lengths to put this together so as to fall under USC 17 fair use exemptions, but I believe this is legal to do, so here you go!
This version of Gears installs and works in Mozilla Firefox, Portable Edition, as well as any non-Administrator account in a normal Firefox installation. This is because it installs into the user profile, NOT system-wide.
It's in "Experimental" status because it's brand new. Please, if you have an addons.mozilla.org account, log in before installing and add review comments. It can only be promoted to public status after it gets positive reviews.
The packaging for this is dead-simple, really: I have one sandbox WinXP system where the GoogleUpdate task is running, updating system-installed Gears binaries. When a nightly script detects a change, it e-mails a notification to me. Then all I have to do is ZIP the Gears Firefox folder, and upload it as an XPI file -- done!
No files in Google's official Gears distribution are altered, but the XPI includes only the files needed to work under Firefox; it completely (and deliberately) omits the IE binaries and the GoogleUpdate.exe app.
If this can get promoted to Public status on AMO, then updated versions will be detected by Firefox's built-in addon update mechanism automatically, no GoogleUpdate required. This is how the extension really should be for Firefox users (and is, for every platform EXCEPT Windows).