Pending on the type of gun, particularly it's finish and how you store it your only worry will be damage to the finish. In a high humidity environment you need to be sure that your gun is:
A: Not blued, by that I mean actual honest blueing of the metal.
B: Not stored in a leather or nylon holster that will attract/retain moisture
C: Secured against theft of opportunity.
D: Has a firing pin safety or is designed as such to have a method of keeping the firing pin from moving until it's supposed to. In the event of a vehicular collision you don't want an inertia caused movement of the firing mechasism
The ammunition should be fine.