WOW! Marty,
It's a good thing that you asked!
First, you can't barbecue on a gas grill. Second, you don't barbecue a tri-tip. (Click into "According to Smoky" from www.barbecuen.com and read the "Glossary" for reasons why.) Third, if you stick a spit in it you will have an unwanted overcooked ring around an ugly hole in the center.
Tri-tip is best broiled or roasted. While in "According to Smoky" read "Pursuit of the Perfect Steak" for broiling technique. A 6 lb. tri-tip is PRETTY LARGE. You may have a sirloin tip, which is about the same. However, I would roast a piece of meat that size at around 350° for about 1 hour until the internal temp. reaches 140° then take it off and let it sit for 10 minutes while the juices set. If you rub it well with a mixture of 1 T. salt, 2 t. each garlic and onion powder, 1 t each ground bay, ground thyme, fresh ground black pepper and put it on a rack over a shallow pan with about 1/2" of water, you won't be disappointed. De glaze the juices while the roast is resting and serve on the side.
Slice the meat thinly across the grain.
Have fun,
Smokey