PaulS:
The NMEAGPS header file defines a class that knows how to read data from, and parse the data from, some specified stream. By itself, you can not do anything to set up the stream.The GPSPort header file is a complete mystery, since you failed to post a link.
But, it is unlikely that you can make the header file do anything to make the GPS output data at a different baud rate. YOU will need to write the code to call the appropriate methods, to do that.
I didn't think to link the library sorry; here it is.