Well, it is:

$ ls -l httrack.tar.gz
-rw-rw-r--    1 roche    roche      778530 aoû 12 
17:29 httrack.tar.gz
$ gunzip httrack.tar.gz
$ ls -l
total 2024
-rw-rw-r--    1 roche    roche     2068480 aoû 12 
17:29 httrack.tar

But your browser (Netscape ? :)) ) may have detected 
the gzipped stream, and decoded on-the-fly without 
renaming the filename.
One of the numerous Netscape bugs, humph.