2006 Chevy Silverado tighten fuel cap warning

There are all sorts of things that can cause that issue. The evaporative emissions system runs a test to ensure no fuel vapors are escaping into the atmosphere. The vehicle simply assumes the gas cap is loose based on the rate of leakage. It could be a leak at the filler neck, tank itself and/or any of the evap valves. This doesnt mean it leaks fuel, but rather fuel vapors from the emissions system.