Based on the way UEFA allocate the games of Team 1 plays Team 4 on such a date while Team 2play Team 3
I am guessing the dates and fixtures will be something like this;
Gladbach at home on Match Day 1 13/14 Sept, then Barca away match Day 2 27/28 Sept, Celtic home Match day 3 18/19 Oct, Celtic away Match day 4 1/2 Nov. Gladbach away Match Day 5 22/23 Nov. Barca Home Match day 6 ,6/7 Dec
Edit, its all bollox as Barca have just tweeted they play us at theres on 19/10 and at the Etihad on 1/11
Gladbach tweeted Etihad fixture 13/9, at theirs 23/11