Nope.
The gun is in your car, the car is on their property, therefore the gun is on their property. And they have the right to say that you can't park on their lot with a gun in the car.
What you're saying is like saying, for example, that you are free to walk into a shopping mall gun free zone with a gun in your pocket because what's in your pocket is yours and does not belong to the shopping mall.
Can you imagine taking a gun on board a plane? "Yes, your honor. I admit to taking a gun on board the airliner, but the gun was in my pocket and it's none of the airline's business what I have in my pocket"
Big to that.
Apples and oranges. There are federal laws against guns on planes, courthouses, banks, etc. These are places where there is a high likelyhood of a gun crime to occur. There are also trained/armed individuals present to protect the public in these situations. Even then, you can still have a gun waiting for you in your car when you leave the protection provided at these places.
Who is going to protect you from some thug on side of the road when you have a flat tire on your way home from work? You left your gun at home because some schmuck says "no guns on my property". Just hope you leave work before I do, because I don't care what any property owner says, I'm well armed. If I see you in trouble, I will gladly help out a fellow citizen.