Sorry, i don't have information about standalone gps STCs for a 733, however over here we are able to get PRNAV approval with newer FMC versions, we upgraded our whole fleet to 10.5/10.6 and will be eligible to fly RNAV approaches (formerly known as gps approaches). Of course that means you need to have sufficient navaid coverage to have good enough ANP (actual navigation performance, which has to be better than RNP of course). Our last 733s came with preparations for gps integration from boeing, however we didn't equip them since we normally have high enough dme coverage, but that could be another way to become enabled for gps approaches.