Der Gegenangriff nach Polen, that is, the counterattack against Poland. I've seen the term used in a Luftwaffe calendar. I thought the same language was used in the history section of Reibert (Der Dienstunterricht im Heere, Service Training for the Army), a training book for enlisted men. It's not, but the text does mention the Gleiwitz, claims that Poland was offered fair terms, and suggests that the German attack was preemptive. For some reason, the Nazis never wanted to be viewed as the aggressor even when they were -- this goes as far as the UK and France being led by warmongers who forced the cruel war on Hitler.
Oddly, Reibert shows a map with the USSR-occupied territories marked but glosses over the fact in the text. Perhaps they were waiting for the final victory in order to decide how they were going to deal with signing a non-aggression pact with the USSR that gave the Soviets half of Poland (which the Nazis could easily have taken for themselves) and then in 1941 declaring war on the USSR... so it is actually difficult to answer that aspect of the WI, without knowing how the Nazis planned to portray the M-R Pact.
The USSR, UK and US would have known the truth; I suspect the German reaction would be to label all of that as propaganda and ban discussing the topic on pain of imprisonment. Some people might know the truth, but the Nazi party line on Poland is not too difficult to accept. The US and UK would view the Soviet invasion as too little, too late -- they were able to gain something of a threshold west of their old territory, but not enough to save the USSR when Barbarossa came and succeeded. The deportation of Poles from the annexed territory might not even be known to the UK and US. The USSR would know the truth, but their voice wouldn't be very strong. Without evidence, the deportations would be treated as rumor. Members of the Polish diaspora in the surviving democracies would try to tell the real story. But post-war relations between the democracies and the Nazis would take priority over the truth.
A handful of Poles might have escaped to the West (most likely via the USSR) with news of the reality of the invasion and occupation. They would insist that there were terrible goings-on at the place the Nazis called Auschwitz, that people were being gassed by the thousands, but again, the voice of the Jewish diaspora would be weaker than the need to appease the Nazis.
Katyn would be fodder for German propaganda and Goebbels would make the most of it. It would be held up as an example before the Vereinte Nationen (the Nazi UN, ITTL) to counter any claims that Germany was needlessly cruel to POWs -- "this is how the Soviets treated their prisoners, and you expected us to follow the Geneva Convention?" The propaganda would make use of shocking photos and footage to erase any doubts.
Would the Polish government-in-exile survive? How would that be dealt with in the UK-German armistice? They would be mentioned in the armistice, I think. My guess is that their best hope is being interned for life in the UK, under something like house arrest. The worst case is they are handed over to the Nazis and their airplane mysteriously crashes into the Channel.