To find the correct gap:
1. Locate your engine's displacement: This is usually found on a sticker under the hood, or in your owner's manual. Common engine sizes for the 1989 Chevy 1500 include 4.3L, 5.0L, 5.7L, and possibly others.
2. Consult your owner's manual: This will be the most reliable source for the correct spark plug gap.
3. Check a reputable online resource: Websites like GM's official site (if they have archival information) or reliable automotive repair manuals (like Chilton's or Haynes) should list specifications for the different engines.
Without knowing the engine size, I cannot give you the correct spark plug gap. Using the wrong gap can lead to misfires, poor performance, and engine damage. Please find your engine size and consult your owner's manual or a reliable resource.