Gulf Islands National Seashore Land Exchange Act (Sec. 3) This bill authorizes the National Park Service (NPS) to convey approximately 1.542 acres of federal land located within the Gulf Islands National Seashore in Jackson County, Mississippi, to the Veterans of Foreign Wars Post 5699 in exchange for approximately 2.161 acres located in the county (the nonfederal land). The values of the parcels to be exchanged shall be determined by an independent appraiser in accordance with nationally recognized appraisal standards. If the values of such parcels are not equal, their values shall be equalized through a cash payment or adjustments to their acreage. The NPS shall require the post to pay or reimburse the costs incurred by the NPS to carry out such exchange. The bill subjects the exchange of the federal and nonfederal lands to valid existing rights. Any nonfederal land acquired by the United States under this bill shall be administered as part of the Gulf Islands National Seashore.