Best thing for you to do is to copy the vin number and bring it to the parts department to the GM dealer and have them print out the PRO codes to what the Sierra came with. I am sure the guys there would be more then happy to give you that information.
So, did you get the truck?
Your codes might have still been in the glove box.
You can use the serial number on a couple different web sites to get the codes, and check to see if it is a real ST.