I think if you change:
SoftwareSerial GPS(2, -1);
to
Stream *GPS = Serial2;
and then change all occurrences of "GPS." to "GPS->" it should work.
I think if you change:
SoftwareSerial GPS(2, -1);
to
Stream *GPS = Serial2;
and then change all occurrences of "GPS." to "GPS->" it should work.