The receiver must be able to handle a -23dBm blocker and still achieve the required sensitivity (in GSM this is -99dBm which is 3dB above normal sensitivity).
If this -23dBm blocker caused compression, the result of loss of gain and increase of noise (the increase of noise is not just down to line-Up gain loss, but to non-linear effects and recprical mixing which cause an additional increase in noise floor) will cause the receiver to lose SNR. A specific SNR is required by the demodulation to guarantee a certain BER dependant on your ADC/demoulation circuit.
So one would normally specify that the each circuit must have a compression point BEYOND the signal level which is induced by the blocker. Often this margin is 3~10dB dependant on how easy it is. So a GSM LNA could have P1dB at about -15dBm (ie a headroom of 8dB) whereas the following mixer might only have an headroom of 6dB because its more tough to design. eventually the combined compression point of the receiver may be somewhere nearer -20dBm.
cheers
aw