Here's the general law in Texas. A person or entity is not liable for the unlawful acts of a third party over whom they had neither the right nor the ability to control. There are exceptions when the person or entity has notice of dangerous conditions that make it foreseeable that a customer would be injured or killed. The classic examples are stop-and-rob stores in high crime areas. Prior crimes at the store are very strong evidence that it was foreseeable that a customer or employee would be killed or injured, but the crimes need not be at that store. They can be in the area, but the frequency and severity of those crimes are also a factor. It's really a jury question.
As more crimes are committed in so-called "gun free zones," it will be easier to argue the company is liable, but note I said "easier," not easy. Since any location can be the target of a hijacker, terrorist or mass-murderer, it would be hard to argue that any specific store should have known they would be a target. If the "gun free zone" was in a high crime area, then that helps, but it's still a steep him to climb in Texas.
If we cannot amend Texas law to add subsection (f) to TPC §30.06, then the next best thing is to provide for civil liability for businesses that post 30.06 signs. Note, I did not include §30.07 signs because 1) that would be unwarranted; and 2) it would make the bill impossible to pass instead of highly improbable.
Chas.
Proposed new TPC §30.06(f) wrote: It is an exception to the application of this section that the property on which the license holder carries a handgun is a commercial business open to the public, or that the licensee would otherwise be considered an invitee on the property.