My tighten gas cap keeps coming on and when goes off my engine light comes on.

I must assume the dealership performed a smoke vapor test on the EVAP system.. That’s the ultimate leak test. I suppose a new leak can occur at any time - what you can do is make certain the gas cap is actually tightened correctly and the rubber seal under the cap is clean and in good shape.. Return to same dealership for further evaluations. Good luck!