Thats just it, unless the law says other wise, you can not win.
You either sue, with out going there and lose.
if you do go there and might have been able to use a gun to protect yourself. By disarming, and going there, you have already agreed to the no gun deal.
I also can't see any law being upheld that says " if you don't let people have guns, you must protect them/ you are liable" standing up to an apeal.
Out side of goverment buildings and airports I really can't think of any place I would go or would need to go, that has a no gun sign. even my wifes work does not have a no gun/weapons sign. no smoking and no using cell phone signs all over the place, but no "no gun sign"