It is gz-compressed, but (1) your browser might decompresses it on the fly after seeing the "Content-Encoding: x-gzip" and (2) most postscript viewers decompress on the fly as gzipped postscript is so prevalent.

    $ curl -s http://www-cs-faculty.stanford.edu/~knuth/fasc6a.ps.gz | dd bs=16 count=1 | xxd

    0000000: 1f8b 0808 689e 1150 0203 6661 7363 3661  ....h..P..fasc6a

> [...] most postscript viewers decompress on the fly as gzipped postscript is so prevalent.

Not only prevalent, but also the right thing to do. PDF was an attempt to (among other aims) achieve smaller filesizes than PS. But that was premature optimization: While a PS file is usually bigger than a PDF, gzipped PS beats PDF.

file fasc6a.ps.gz fasc6a.ps.gz: gzip compressed data, was "fasc6a.ps", from Unix, last modified: Thu Jul 26 12:45:44 2012, max compression