FPGS stands for "Fine Grain P-States" which is a new feature of the power management and allows frequency variance in 25 MHz increments. The power management unit built into the SoC controls the FGPS of the cores which controls the frequency of the cores to take into account workload and things such as thermal characteristics and power being consumed by the processors.
The Power Management Unit (PMU) monitors the system characteristics and the workload that the system is operating under and can adjust the frequency of the cores to make accommodations to execute the workload with the minimum amount of power. As an example, it is better in regards to power consumption, to increase the core speed of the cores currently operating to meet the workload requirements than to initiate and power up idle cores. This is reflected in the Reduced Core Boost specification versus the All Cores Boost frequency.
All the controls of the PMU are automatic and do not require any control or input by the customer. You can via the BIOS settings make changes to the C-states (See BIOS guide) and Global C-States, but FGPS' are controlled automatically by the PMU.