In my experience, a gallon is a ton of paint. I may have used about a quart so far. That quart covered my entire tub, 4 times! I still have a bunch of small parts to paint yet (hood, tailgate, doors, windshield frame, grill. I'm shooting DuPont Imron 5000 which is mixed 3 to 1 (paint to catalyst).
If you've got a paint shop close by I'd recommend buying a quart at a time.