So, wait, you're homeschooling but you want into the school's GT program? Am I following you? Is it an afterschool program? Or are you looking for partial homeschooling? I'm not familiar with MS law: what exactly is mandated by the state? Are you sure it includes homeschoolers?
Schools often limit who can and cannot be in GT programs because GT programming is typically an unfunded mandate. The schools can't afford to let everyone in, often not even every GT child. You may have a real fight on your hands if their rule says that kids must be enrolled full-time to be in the program.