Manual:$wgUseGzip

Details
This causes the server to send pages in to the browser in a compressed format in order to save bandwidth, with a resultant increase in CPU usage. When using the file cache, we can store the cached HTML gzipped to save disk space. Pages will then also be served compressed to clients that support it.

THIS IS NOT COMPATIBLE with ob_gzhandler which is now enabled if supported in the default LocalSettings.php! If you enable this, remove that setting first.

Requires zlib support to be enabled in PHP.

Between 1.2.0 and 1.3.0 (inclusive) the default value was true if the function 'gzencode' is defined, otherwise false. For prior and later versions it defaults to false.

It appears that $wgUseGzip returns garbage HTML in combination with a PHP Accelerator and $wgUseFileCache.