In a medium bowl, beat together the cream cheese, butter, shredded mozzarella, and garlic salt until well combined. Set aside.
Lay loaf of bread on a cutting surface with a chopstick on either side of it for guidance (if you don't have chopsticks, anything similar can be used that is roughly ¼ of an inch thick--skewers, pencils, wooden dowels, etc). Cut bread into ½-thick slices, stopping at the chopsticks to avoid cutting all the way through.
Spread roughly 2 tablespoons of cheese mixture in between each slice of bread. You don't need to be worry about neatness.
Wrap loaf of bread in foil and place over indirect heat on grill for 20-25 minutes, or until cheese is melted and gooey. Alternatively, bake well wrapped loaf in a 350° F oven for 20-25 minutes. Serve immediately. Any leftover bread may be re-wrapped and stored in the fridge. Reheat in a 300° F oven for 30 minutes or until cheese is melted.