It is not a gzipped file; open with any postscript viewer.
$ 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..fasc6aNot 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.