The fact sheet is pretty informative, but notes things are subject to change based upon as yet unimplemented final regulations. Couple of key points, GINA sets a federal floor of protection. Where state laws provide more protection employers and insurers must comply with them also. Second, "GINA generally will prohibit discrimination in health coverage and employment on the basis of genetic information. GINA, together with already existing nondiscrimination provisions of the Health Insurance Portability and Accountability Act, generally prohibits health insurers or health plan administrators from requesting or requiring genetic information of an individual or the individual’s family members, or using it for decisions regarding coverage, rates, or preexisting conditions. The law also prohibits most employers from using genetic information for hiring, firing, or promotion decisions, and for any decisions regarding terms of employment."
The New York Times has a pretty good article on the Act.