2002 Chevy Blazer Intake manifold gasket

Engine problem
2002 Chevy Blazer 70000 miles

Hello. My daughter's check engine light came on and she took her car to the local GM dealership where they told her that her gas cap needed to be tightened, that's why the sensor was on. They also told her that the cars intake manifold gasket needed to be replaced for 850.00.I want to make sure she is not being taken advantage of. Thanks so much