Several years ago I read a book about the days leading up to WWII. The Gleiwitz incident had its own chapter.
Recently I've had fellow boardmembers (at various WWII history and collecting boards) and a student questioning the both the reality of the Gleiwitz incident or, in one case, claiming it really was the Poles. There seems to be a lot of sites like THIS supporting this and other Germany-innocent-to-WWII arguments.
One of the arguments used is that Naujocks is the sole source regarding the german role and that he's not a very trustworthy source.
The sites also use spurious arguments like: "The Naujocks testimony is a hoax and it is being revealed right here. The American interrogator “Lieutenant Martin” would not use the word “fortnight” for two weeks. 99% of Americans don’t even know what it means." Etc.
So what documents/facts are there supporting the reality of the Gleiwitz incident?
The link you provided includes an article by Carolyn Yeager — who is a known neo-Nazi (she was the leader of the Texas branch of the American National Socialist Workers Party, among other things) and Holocaust denier. So that should give you some sense of her intellectual credibility. It also includes an excerpt from a book by Robert Smyth, published by Steven Books — who appear to specialise in right-wing revisionism, with a particularly extensive line in Holocaust denial and Nazi apologist. So again, not a particularly solid evidentiary basis.
It's not a very scientific methodology, but the first page of Google results for 'Gleiwitz hoax' includes the post you linked to, more from Yeager and this post by Carlos W. Porter, who also has a reputation as a Holocaust denier.
Gleiwitz and the wider false flag operation, Operation Himmler, are recurring features in pretty much all of the historiography, and I've never seen anything in the scholarship that casts serious doubt on that narrative. I can't find any credible scholarship arguing that it was anything other than a provocation designed to provide Hitler with a casus belli for invading Poland.
It looks like the primary argument the Gleiwitz 'truthers' make is that Naujocks' Nuremberg affidavit is the main primary source document on Gleiwitz, along with ambiguous claims that it was "probably signed under duress". But Naujocks wasn't the only witness to testify that Gleiwitz was a provocation — Erwin von Lahousen, a senior officer of the Abwehr (military intelligence) also testified to that effect.
It's a pretty threadbare case with which to argue against the entire mass of Second World War historiography. The leading scholarly biographers of the attack's organisers — Himmler (Peter Longerich, Heinrich Himmler: A Life) and Reinhard Heydrich (Robert Gerwarth, Hitler's Hangman) — both present Gleiwitz as a settled matter.
My overall impression: it's revisionist nonsense intended to absolve the Nazi regime of responsibility for starting the war.