I am not sure why you would use Gutenmark to produce HTML files from books for which there are HTML files. It is those books that are the subject of my tutorial.
Please keep in mind that there are many features of the original printed books that are not or not adequately preserved in the TXT file, such as illustration, all-caps, bold print, italics, non-ASCII characters such as wide dashes and curly quotes, et cetera.
My tutorial was written for people who like lots of control, and who wish to have as good an output as possible. I have already written a follow-up article that explores working from TXT files, and that will mention the various automatic conversion tools, of which Gutenmark is only one.