The first mention I've been able to found is this article in The Boston Globe, 02.10.1945, p. 10:
It cited an affidavit by some Michael Geissler who tells the story we know now, from hearsay.
This was further mentioned during the Nuremberg IMT, when Erwin von Lahousen from the Abwehr testified on 30.11.1945 about procuring the Polish uniforms for some secret goal, which he speculated to have been the Gleiwitz incident (IMT, vol. 2, pp. 450-1, https://www.loc.gov/rr/frd/Military_Law/pdf/NT_Vol-II.pdf ).
On 06.12.1945 several documents were introduced in order to confirm Lahousen's testimony, most importantly the document 795-PS, a conversation of a high-ranked but then unnamed author (later established to be Canaris) with Keitel on 17.08.1939 (IMT, vol. 3, p. 235, https://www.loc.gov/rr/frd/Military_Law/pdf/NT_Vol-III.pdf ; the document itself is in vol. 26, pp. 336ff, https://www.loc.gov/rr/frd/Military_Law/pdf/NT_Vol-XXVI.pdf ):
I reported my conference with Jost to Keitel. He said that he would not pay any attention to this action, as the Fuehrer had not informed him, and had only let him know that we were to furnish Heydrich with Polish uniforms.
The uniforms were thought to be for various provocations including the Gleiwitz incident.
And the complete confirmation of the fakery was publicized together with the affidavit of the organizer of the "incident" - the SD-man Alfred Naujocks on 20.12.1945 (IMT, vol. 4, pp. 242ff, https://www.loc.gov/rr/frd/Military_Law/pdf/NT_Vol-IV.pdf ).
Naujocks testified:
On or about 10 August 1939 the Chief of the Sipo and SD, Heydrich, personally ordered me to simulate an attack on the radio station near Gleiwitz, near the Polish border, and to make it appear that the attacking force consisted of Poles. Heydrich said: 'Actual proof of these attacks of the Poles is needed for the foreign press, as well as for German propaganda purposes.' I was directed to go to Gleiwitz with five or six SD men and wait there until I received a code word from Heydrich indicating that the attack should take place. My instructions were to seize the radio station and to hold it long enough to permit a Polish-speaking German, who would be put at my disposal, to broadcast a speech in Polish. Heydrich told me that this speech should state that the time had come for the conflict between the Germans and the Poles and that the Poles should get together and strike down any Germans from whom they met resistance. Heydrich also told me at this time that he expected an attack on Poland by Germany in a few days.
[...] Müller stated that he had 12 or 13 condemned criminals who were to be dressed in Polish uniforms and left dead on the ground at the scene of the incident to show that they had been killed while attacking. For this purpose they were to be given fatal injections by a doctor employed by Heydrich. Then they were also to be given gunshot wounds. After the assault members of the press and other persons were to be taken to the spot of the incident. A police report was subsequently to be prepared.
[...] Müller told me that he had an order from Heydrich to make one of those criminals available to me for the action at Gleiwitz. The code name by which he referred to these criminals was 'Canned Goods.'
[...] The incident at Gleiwitz in which I participated was carried out on the evening preceding the German attack on Poland. As I recalls war broke out on the 1st of September 1939. At noon on the 31st of August I received by telephone from Heydrich the code word for the attack which was to take place at 8 o'clock that evening. Heydrich said, 'In order to carry out this attack, report to Muller for "Canned Goods."' I did this and gave Müller instructions to deliver the man near the radio station. I received this man and had him laid down at the entrance to the station. He was alive, but he was completely unconscious. I tried to open his eyes. I could not recognize by his eyes that he was alive, only by his breathing. I did not see the shot wounds, but a lot of blood was smeared across his face. He was in civilian clothes.
[...] We seized the radio station as ordered, broadcast a speech of 3 to 4 minutes over an emergency transmitter, fired some pistol shots, and left.
Indeed, the unfortunate fellow was the Pole Franciszek Honiok, earlier arrested specifically for this operation. As we see from the mention of the civilian clothes, the uniform was not used in this case, despite Müller's alleged instructions. According to Naujocks' 1963 interview:
That's what we talked about at first. But I could make it clear to Müller that we could not use a uniformed man as a dead man in Gleiwitz. "Fine," Müller said, "then you'll get him in plain clothes."
That the uniforms were indeed ordered is further confirmed in Halder's war diary, published later, in which on the same date (17.08.1939) we read (quoted from Winfried Baumgart, "Zur Ansprache Hitlers vor den Führern der Wehrmacht am 22. August 1939. Eine quellenkritische Untersuchung", Vierteljahrshefte für Zeitgeschichte, 1968, 16(2), p. 157n71, https://www.ifz-muenchen.de/heftarchiv/1968_2_2_baumgart.pdf ):
Canaris: Himmler - Heydrich Obersalzberg 150 Polish uniforms with accessories (Dr. Trumler). Upper Silesia.
It is thus possible, if Naujocks' claim above is correct, that either their use was canceled at the last moment, or they were to be used for other related "incidents".
In 1962 the historian Jürgen Runzheimer published an important article for which he interviewed the still living relevant German witnesses (like various policemen from Gleiwitz) and in which he used an informative unpublished report of the head of the Gleiwitz station written in 1949 based on the reminiscences of the station's personnel. On this basis he tried to reconstruct the "incident" in detail (see "Der Überfall auf den Sender Gleiwitz im Jahre 1939", Vierteljahrshefte für Zeitgeschichte,1962, 10(4), pp. 408–426, https://www.ifz-muenchen.de/heftarchiv/1962_4.pdf ), though not everything he got correct. Important details, like the identity of the victim, got cleared up only later.
Runzheimer was skeptical of the use of uniforms in relation to Gleiwitz for various reasons and pointed out, that Naujocks and his men entered the building in civil clothes. That said, the documents about the uniforms still prove the intent to provoke, if not in Gleiwitz, then elsewhere. And the participants' and other witnesses' testimonies are clear that there were, indeed, no "Polish terrorists", only the Nazi ones.