Make the rub: combine the garlic powder, salt, thyme and sage in a bowl.
Rub the spice mix into the ribeye steak. Let the steak sit at room temperature with the rub 10 minutes prior to grilling.
Preheat a grill to medium high.
Coat the mushrooms with oil spray and place on the grill. Grill for 4 to 5 minutes, then turn over to grill for another 4 to 5 minutes until the mushrooms are cooked through (they should be soft and warm on the inside).
Remove the mushrooms from the grill, and set aside to cool. When cooled, cut the mushroom across in long, 1/2 inch strips. Gently toss the mushrooms slices with the olive oil and balsamic, and set aside.
You can start grilling the steak as the mushroom cooks. Grill the steak on one side for 6 to 7 minutes, then flip to grill for another 6 to 7 minutes. Cooking time depends on your preference and thickness of steak, but 12 minutes of total grilling time should be good for medium rare, a bit longer if you like it cooked more.
Remove steak to a platter when it is cooked to your liking. Let it rest for a few minutes, then slice the steak crosswise.
To serve, plate the steak and mushrooms slices together, alternating between steak and mushroom slices. Enjoy!