This is a Super Easy Solve on Site (SESOS) puzzle. This area can be quite busy with muggles during daytime hours. Please use an extra measure of stealth while searching for this cache. Also, please return container exactly where you found it.
Find the 4 digit number on the northwest box in the grouping of boxes at GZ. Checksum for these 4 numbers is 15. Assign A thru D to each of the numbers, left to right. Then, solve for the last 3 digits of both north and west coordinates.
N (B + D) (A + D - C) (D + C + B)
W (C + D) (D + B) (D + A - B)