With assistant coach Marcus Sorg again taking charge of the side as Joachim Low recovers full fitness, Manchester City midfielder Ilkay Gundogan played in Reus to fire the opening goal after 10 minutes.
Leroy Sane's run set up Gnabry for a second in the 17th minute, before Leon Goretzka swiftly headed in Joshua Kimmich's cross.
Gundogan made it 4-0 from the penalty spot after Joonas Tamm had fouled Goretzka.
Borussia Dortmund's Reus added another ahead of the break with a fine 25-yard free-kick.
It remained one-way traffic in the second half.
Gnabry scored a sixth when he turned in Marcel Halstenberg's cross on 62 minutes, with Sane then seeing his effort ruled out for offside.
Substitute Timo Werner slotted in number seven with 10 minutes left, and there was still enough time for Sane to get on the scoresheet, curling in a left-foot shot in the 88th minute.
Die Mannschaft now have three wins out of three matches so far, and are second behind leaders Northern Ireland, who have played a game more.
Germany's next Euro 2020 qualifier is against Nations League runners-up Holland in Hamburg on September 6 when Low is expected to be back in the dugout.