Hello.
Power saving modes are most useful when vehicle is stationary, so that it could conserve energy of itself and vehicle's as external power source is not charging at that moment. When vehicle has ignition then it can charge its battery, and device also charges its battery of it.
There is no command to turn off GPS (aside from sleeping modes) functionality as it its main function. But You can send SMS to change configuration, so You can disable record generation by changing Min Period(also there are Min distance, Min Angle and other parameters that work when vehicle is on the move) value to 0 it will effectively prevent device from generating new record.
Is there are any reason why You do not want GPS?
Best Regards