Hello,
This post is similar with #114
BUT, it is modified according to new understanding after having examine and get advices from many experts.
The reason we post it to here, is to confirm that we understand the matter correct, because we are newbies to Ghostscript.
So we simplify the whole matter to the following, and we will appreciate your comments:

I have a pdf file (named input.pdf) and i want to convert it to a new pdf file (named output.pdf) using Ghostscript.
The reason/purpose i want to convert it is only one - to remove all its old metadata (classical and xmp).
So, i apply the command:

gsc.exe -o output.pdf -sDEVICE=pdfwrite input.pdf pdfmark.txt

With the following pdfmark.txt:
[ /Title ()
/Author ()
/Subject ()
/Creator ()
/ModDate ()
/Producer ()
/Keywords ()
/CreationDate ()
/DOCINFO pdfmark

Please, very kindly i ask just to confirm that the above command will do the job i want that is :
It will remove all its old metadata classical and xmp and the newly created file will have its new metadata but there will be no trace of the old file metadata.
(I know that the only exception is that the Producer name will be Ghostscript, and the Creation Date will be applied during the creation of the converted file and can not be changed), - and we are happy with this.

We will appreciate very much your comments regarding if the above command will do the job we want.
Thank you very much!