The White House, Congress and Federal courthouses are "gun free zones", but they also have armed security guards and metal detectors amd x-ray machines. The Congress does not depend on signs alone to protect itself. There have been armed attacks at the Capitol before. The old District of Columbia laws against gun ownership did.nothing to protect the Capitol. Congress instead built a new entry system to screen all visitors.
We know how to keep killers with guns out of government buildings. If we give schools and other public venues the same physical protection that the President and Congress and judges give themselves, then we can prevent attacks. Signs without security just invite shooters.