Preheat an outdoor grill for medium-high heat and lightly oil the grate.

Generously season pork tenderloin with garlic salt and pepper.

Cook pork on the preheated grill, turning twice on each side, until no longer pink in the center, 6 to 7 minutes per side. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C). Remove pork from grill and cover with aluminum foil; let rest for 5 minutes. Slice pork and place on a serving platter.

Melt butter in a skillet over medium heat; add apples. Cover skillet and cook for 5 minutes. Stir apples and sprinkle with sugar; reduce heat to low. Cover skillet and cook apples for 5 more minutes. Remove cover; cook and stir until apples are lightly browned, 2 to 3 more minutes. Spoon apples over sliced pork.