Head coach Hansi Flick confirmed earlier on Tuesday that Gnabry would likely be absent from Wednesday's game at the Allianz Arena after complaining of a sore throat.
Bayern later announced the 25-year-old had tested positive for COVID-19.
Serge Gnabry has tested positive for Coronavirus. The FC Bayern forward is feeling fine and is now quarantining at home. ℹ️ pic.twitter.com/AAnzZ5HtLK— 🏆🏆🏆FC Bayern English🏆🏆🏆 (@FCBayernEN) April 6, 2021
The holders were already missing star striker Robert Lewandowski due to a knee injury, leaving them without an attacking duo who have been directly involved in 63 goals in all competitions this season.
PSG, meanwhile, had already confirmed Marco Verratti and Alessandro Florenzi would miss the first leg after returning positive coronavirus tests.
Bayern defeated the Parisians 1-0 in last season's Champions League final, with Kingsley Coman scoring the winner.
Should they avoid defeat in the first leg, Bayern would become only the second team in the competition's history to go 20 consecutive games unbeaten, after Manchester United (25 between 2007 and 2009).
However, they have only won four and lost five of the previous 10 meetings in this tournament with the Ligue 1 champions. Only Real Madrid (10) have beaten Bayern more often in the competition.