Make the bruschetta: Grill the bread on an oiled rack set about 4 inches over glowing coals for 1 minute on each side.
Make marinade: In a small bowl whisk together marinade ingredients. Put half of mushroom caps and half of marinade in each of 2 large sealable plastic bags, arranging mushroom caps in 1 layer, and seal bags, pressing out excess air. Marinate mushrooms at room temperature, turning bags over several times, at least 1 and up to 2 hours.
Prepare grill.
Remove mushrooms from marinade and grill 2 to 3 minutes on each side, or until tender. (Alternatively, mushrooms may be grilled in a hot well-seasoned ridged grill pan over moderately high heat.) Transfer mushrooms with tongs to a cutting board and slice thin.
Spread 1 side of bread with gorgonzola butter.
Arrange warm mushrooms on toasts and spoon vinaigrette over each bruschetta. Garnish with thyme.