Gnabry's diving header opened the scoring following Joshua Kimmich's clever dinked-through ball.
Kimmich provided another assist with an excellent ball which Coman slotted past Hertha goalkeeper Manuel Riemann. Borussia Dortmund, who drew 1-1 with relegation-threatened VfL Bochum on Friday, are now a point behind the leaders.
Anthony Losilla gave VfL Bochum the lead after five minutes with Karim Adeyemi levelling soon afterwards for Dortmund, putting Bayern back in the driving seat with four games to go. Dortmund are vying to win the Bundesliga for the first time since 2012.