In fact control system is trying to control SRV downstream gas pressure FPG2 or P2 (upstream pressure of the GCV) by moving SRV, not just controlling the valve.
In order to get closer to the FPGR (P2 setpoint) which is a function of TNH (Turbine speed). FPGR is lower at low speed in order to provide better flow control at low fuel flow. At the startup P2 increase linearly to the 100% rated speed setpoint.
The downstream pressure of the GCV is a function of CPR and fuel nozzle size. FPGR must be sufficient to maintain critical pressure drop across the control valves at max flow, the P2 and the critical pressure drop design of the GCV ensures that the percentage valve stroke is proportional to percentage fuel flow.
Sorry for any misunderstanding of your question.