Trim and discard fat from beef. Wipe meat with a damp towel, then sprinkle with garlic salt and pepper.
Prepare barbecue for direct heat.
If using charcoal briquets, cover firegrate with a single, solid layer of ignited coals; let them burn down to desired heat. Set grill in place and measure heat.
If using a gas barbecue, turn all burners to high and close lid for 10 minutes. Adjust burners to desired heat. Set grill in place and measure heat.
When grill is very hot (you can hold your hand at grill level only 1 to 2 seconds), lay meat on barbecue. Cook, turning as needed to brown evenly, until meat is rare in center of thickest part (cut to test) and 125° to 130° on a thermometer, about 20 minutes, or until it has reached desired doneness.
Transfer the tri-tip to a board and cut meat across the grain into thin, slanting slices.