Using a sharp knife, cut small slits into the top of the roast. Stuff the slits with slices of garlic.
Mix together salt, pepper, & garlic salt. Rub the entire mixture all over the tri-tip. Refrigerate at least an hour & up to all day. Take the meat out of the refrigerator about 20 minutes before you are ready to grill.
preheat an outdoor grill for high heat.
Place the meat directly above the flame for 5 to 10 minutes per side (depending on the (thickness) to sear the meat & lock in the juices.
Turn the grill down to medium heat & continue to cook for another 25 to 35 minutes, trying not to flip it too much. Check the doneness with a meat thermometer. The thermometer should read at 145 degrees F for medium-rare. Let stand, covered loosely with aluminum foil, for 5 minutes before slicing.
