GPS Verifier is a verification system that is used on some SASSIE systems to confirm that you were at the correct shop location, at the time that you were supposed to do the shop.  A GPS Verifier code contains a datestamp and location (based on your phone's GPS) where you were when you got the code.  You can get a GPS Verifier code using the GeoVerify app (available for iPhone and most Android phones running Android 2.2 and beyond).  

Where can I get the GPS Verifier app?

Do I need a GPS Verifier code to complete my SASSIE shop?

I did a shop but I didn't get a GPS Verifier code - can I go back and get it later?