Gleiwitz incident, rather than being a one-off incident affirming foreign intervention into German affairs, was sold to be a part of a series of outside interventions. News of persecution of Germans living within Poland were already widespread by early 1939 and Hitler himself had made public declarations as to what the Germans viewed as Western hostility.

If they [the Western Allies] expect the Germany of today to sit patiently by until the very last day while they create satellite States and set them against Germany, then they are mistaking the Germany of today for the Germany of before the war.

The view promoted by the German state was one of Germany under siege. The continued cat and mouse as demands, annexation, and treaties spread throughout Europe; guarantees such as to Greece, Yugoslavia were seen as the west improperly exerting its control. Limitations on German production of the navy such as the Anglo-German Naval Agreement were promoted as limiting the defense of Germany itself against the one-sidedness of British production (capping tonnage of the Germany navy to 35% of the British Navy). One of the direct challenges to Germany came in March 1939 when Britain proclaimed support for the Polish government against German aggression; Hitler used this opportunity to display that Germany was uninterested in aggression, instead the movement of foreign troops was worrying to German defense.

The worst is that now Poland, like Czechoslovakia a year ago, believes under pressure of a lying international campaign, that it must call up troops, although Germany has not called up a single man and had not thought of proceeding in any way against Poland.

Hitler was lying, as by this time several calls and reports within the German govermnent had been drawn preparing for war, specifically enough, by September 1st 1939. The Gleiwitz incident as we know occured on August 31st. This direct claim also lead to Poland delaying mobilizing its forces on August 29th.

As to the extent of that being accepted among the German people themselves, the historian William L. Shrier notes his observations and opinions in "The Rise and Fall of the Third Reich."

To anyone in Berlin that weekend when March 1939 came to an end, as this writer happened to be, the sudden British unilateral guarantee of Poland seemed incomprehensible, however welcome it might be in the lands to the West and East of Germany. [...]

Yet later on September 1st, he strikes a different tone while still illuminates the general attitude and propaganda of the time.

Perhaps, it occurred to me, the German people were simply dazed at waking up on this first morning of September to find themselves in a war which they had been sure the Fuhrer somehow would avoid... What a contrast one could not help thinking, between this grey apathy and the wy the Germans had gone to war in 1914. Then there had been a wild enthusiasm... There was no such demonstrations this time for the troops or for the Nazi warlord, who shortly before 10AM drove from the Chancellery to the Reichstag through empty streets to address the nation on the momentous happenings.

Source: The Rise and Fall of the Third Reich. Book 3. Chapters 14-17.