Preheat the oven to 350 degrees F (175 degrees C).

Combine rosemary, garlic salt, thyme, and pepper in a large resealable plastic bag. Place pork loin in the bag, seal, and toss until thoroughly coated with spice mixture. Transfer to a medium baking dish.

Bake in the preheated oven until cooked through, about 1 hour. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C).