A mere 26 days after winning the UEFA Champions League, Hansi Flick's side showed no signs of tiredness and powered into a fourth-minute lead thanks to Gnabry's superb strike.
That was added to by Leon Goretzka after 19 minutes before Robert Lewandowski swept home a third from the penalty spot shortly after the half-hour mark.
Gnabry completed his hat-trick with two goals inside the opening 15 minutes of the second half before Thomas Muller, Leroy Sane – making his debut after his July move from Manchester City – and 17-year-old Jamal Musiala completed Schalke's humiliation.
Bayern started in blistering fashion as Gnabry controlled Joshua Kimmich's long pass over the top, turned away from his marker with a deft turn and whipped a superb left-footed strike past Ralf Fahrmann from 18 yards.
Flick's side bamboozled Schalke's backline with relentless pressure and the hosts were unsurprisingly undone again shortly before the midway point of the first half when Goretzka slotted home from the edge of the penalty area after being picked out by Muller.
Lewandowski, who scored 55 goals in all competitions last season, opened his account for the 2020-2021 campaign from the spot in the 31st minute after he had been brought down by Ozan Kabak, while Niklas Sule crashed an effort against the post shortly before the interval.
Bayern started the second half like it did the first, Gnabry slotting into an empty net two minutes after the restart after Sane had pushed the ball past an on-rushing Fahrmann.
Sane was the provider again after 59 minutes, teeing up Gnabry to seal his hat-trick from 10 yards, while an outrageous rabona from Lewandowski set up Muller to add a sixth in the 69th minute, the 31-year-old's side-footed volley squirming underneath Fahrmann.
Sane capped a fine display with his first Bayern goal two minutes later, slotting past Schalke's woefully exposed goalkeeper after being played through by Kimmich, while substitute Musiala scored his first Bayern goal nine minutes from time to complete the nightmare for Schalke.