Preheat the smoker to 250°F. Set it up for indirect cooking so the meatloaf can cook steadily without sitting over direct heat.
In a large bowl, stir the panko and milk together. Let them stand for 2 minutes so the breadcrumbs soften.
Add the egg, chopped green onion, garlic, ginger, soy sauce, sesame oil, and black pepper. Mix until combined, then add the ground beef. Gently mix just until the seasonings are evenly distributed. Avoid packing the mixture too firmly.
Shape the mixture into a compact loaf and place it on a smoker-safe tray. Smoke until the center reaches 155°F, checking the temperature with an instant-read thermometer.
While the loaf smokes, combine the ketchup, gochujang, brown sugar, and rice vinegar in a small bowl. Stir until smooth.
Brush the glaze over the meatloaf when its center reaches 155°F. Continue smoking until the center reaches 160°F and the glaze looks glossy and set.
Carefully remove the meatloaf from the smoker and let it rest for 10 minutes before slicing. Garnish with the reserved green onion and sesame seeds.