We have gpif_examples (gpiftostorage) in the firmware folder of FX3 SDK. You can refer to them for your implementation.
This example illustrates the use of the FX3S firmware APIs to allow an external processor to access SD/MMC devices through the GPIF port. A 16-bit Async. ADMux interface is used on the GPIF side, and the P-port mailbox registers are used to receive control requests from the external processor. The firmware interprets the various requests received, and initiates data transfers that are requested by the processor.