I am still interested in what you would suggest. I don't "know" that no one is GF, but it is not known if they are. As far as I know the place we are getting the food from does not have any GF options.
When you send out the invitation, you ask people to RSVP and to let you know if they have any dietary needs. If someone does have a specific need, you check with the place you are ordering from to see if they have GF options (NEVER assume or think you know). If they don't, you may need to consider ordering something for them from other places. Of course, you can just tell them there will be nothing for them, but that's not really the holiday spirit or a way to take care of employees.