This inspector attempts to decode the HTTP response body as a GZip stream. To use, download this zip file and unzip it in the "Inspectors" folder in your Fiddler installation, and then restart Fiddler. A "Gzip" tab will appear if everything works out OK.
NOTE: GzipInspector is no longer maintained. Reportedly, it does not work anymore on newer versions of Fiddler.
So it seems, but I never figured out how to use it.