The Phantom Manual states GPS/Atti/Atti. As Olof said, unless it is a printing mistake, we normally see on the 3 position, GPS/Atti/Man.
Difference between GPS and Atti is.... GPS mode is just that, it is more of a GPS lock and Atti mix, while in that mode, the aircraft should maintain a relatively good altitude (Atti) and a good position hold (GPS). Whilst just in Atti would hold a certain Altitude. Atti is activated while the switch is in the middle and your throttle is at around 50% give or take +/-5% I think.
I read in the manual, the compass is mounted to the landing gear, I take it you have that part? Calibrated and all?