Full thread here:
http://groups.google.com/group/gears-users/browse_thread/thread/13ad3b66d5b0ef99/

Key snippets:

On Fri, Jul 11, 2008 vvpreetham@gmail.com <vvpreetham@gmail.com> wrote:
>
> Is there (will there be?) a Gears API that allows compression of
> "request" content in the browser before sending (POST) the request to
> the server. I see a great use for this now that Gears enables us to
> use browser side db (sqlite). We do a lot of data sync-ups of content
> from the Gears db back to the server and enabling compression of the
> payload potentially helps increase performance of the POSTs.
>
> Note that I am requesting an inverse scenario of the already existing
> content-encoding: gzip, mod_deflate (similar) features.

On Mon, Jul 14, 2008 Michael Nordman <michaeln@google.com> wrote:
> This is a great feature request.
>
> ghr.setHeader('Content-Encoding', 'gzip');   ----> compressed body when
> POST'ing