Yes, INC , suitability is mandatory for GNM course. All nursing college must be approved by INC before they offer nursing courses. The Indian  Nursing Counsil approval is important as it ensures that college meets the required standards and guidelines set by the council. If your college is approved by state but not by the INC , there mat be certain challenges you might face. You might face challenge for higher studies,and job opportunities as many institutions and various  hospitals and clinics prefer candidates who have passed from INC approved colleges.