I don't understand what happens if Costello catches Gabriel in the dressing room. All I gather is that Costello screams, and somehow Gabriel is dead. I don't get how that works.

They stop Gabriel from being in the auditorium when Von Glower is on tilt, and to expose Von Glower, Gabriel had to be in the auditorium.

So Von Glower goes undetected, Gabriel remains a werewolf (now with no hope of killing Von Glower), and Gabriel has to either kill himself like Ludwig did or commit murder like Von Zell did.