There is nothing like positive power or negative power.
The negative sign signify the LOSS in system and the positive sign signify the GAIN in system.
Now as far as GSM is concerned.
Let us assume that Transmitter is transmitting at 2W. W=watt which is the absolute power.
Then at Receiver the power received will be less then 2W, because of loss*. let us say 1W.
Just look at this picture in terms of dBm.
Here reference is 1mW.
You will measure the received power wrt 1mW.
Ease to work with dBm unit in terms of calculations , graphs etc is the reason that wireless engineers prefer dBm.