I guess your confusion comes from the term “attenuated maximum GMSK output power” ?
If yes, then keep in mind that the TRX of one cell are going to transmit at full blast, by default. However if you mix different generations of TRX (for ex. a 40dBm trx with a 43dBm trx within the same cell), then the 43dBm trx will be automatically attenuated to 40dBm.
The actual “attenuated maximum GMSK output power of the BTS” for this cell is, in this scenario : 40dBm.
They call it “attenuated” for this reason : some TRX may be attenuated in order to have the exact same power in all TRX of a cell.