If you ban guns on your property, and include your parking lots, you are in effect forbidding your employees from carrying anywhere on the days they work - including on their trips to and from work.
IMO, you should be able to forbid your employees from carrying on the premises. But if you do so, you should be required to provide secure storage for their firearms while they are on the premises.
Should employers also be forced to provide freezers for people who come to work with an ice cream cone, which they are not allowed to take into company premises?