Porting GPIO code for PPS from rtos to linux

Has anyone try to port the Time Synchronization code from rtos to linux?

In the DJI sample, most GPIO related code is under platform/rtos_freertos.  We are trying to port these source files into platform/linux, but there are so many source files and definitions that seem to be hardware dependent.  DJI support suggests to learn about linux GPIO handling and import only the linux GPIO driver-related code, but we have a hard time to decide where to start. 

Any pointers are greatly appreciated.