Using a sharp knife, cut small slits into the top of the roast. Stuff the slits with slices of garlic.
Mix together salt, pepper, and garlic salt. Rub entire mixture all over the tri-tip. Refrigerate at least an hour and up to all day. Take the meat out of the refrigerator about 20 minutes before grilling.
Preheat an outdoor grill for high heat.
Place the meat directly above the flame for 5 to 10 minutes per side (depending on thickness) to sear the meat and lock in the juices.
Turn the grill down to medium heat and continue to cook for another 25 to 30 minutes, trying not to flip it too much. Check for doneness with a meat thermometer. Thermometer should read at least 145 degrees F (63 degrees C) for medium-rare. Let stand, covered loosely with aluminum foil, for 5 minutes before slicing.