Hi Aso
Nice question
There is no fixed relation as how much the 8-PSK power level will be lower than GMSK power level.It is assumed that The maximum average output power for 8-PSK is 4 dB lower compared to GMSK if the transceiver unit is configured with maximum output power.
In Nokia & Ericsson ,there is no parameter by which we can regulate the 8-PSK power level.In Huawei ,we have a separate attenuation parameter to do this,but it is recommended not to use it as it might lead to inaccurate MS meaurements.