You should consult your owner's manual for the precise specification. If you don't have the manual, you can try:
* Looking at the existing spark plugs: Sometimes the gap is stamped on the plug itself, or you can compare the existing gap with a spark plug gapping tool. *Caution:* Don't rely solely on this; the previous gap might not be correct.
* Checking a repair manual: A Haynes or Chilton repair manual specific to your year and engine will have the correct specification.
* Checking online resources: Search online for "[Your engine displacement] spark plug gap" along with "1993 GMC Vandura". However, always double-check this information against a reliable source like a repair manual.
Incorrect spark plug gap can lead to misfires and damage to your engine. It's crucial to get this right.