You have a lapse in your employment history? No, you don't; you worked at a confidential organization on a project that you are not at liberty to discuss because you are honoring the non-disclosure clause in your employment agreement.
You helped a family member or worked on sorting out your backyard or restored an old motorcycle or built some furniture or took some classes to get a certificate or painted a room or whatever. If you say it's confidential, it's confidential and it's a project so what you say is true.