How does FFMpeg call the Gstreamer encoding plugin on Jetson TX2?

For some demands, I need to use FFMpeg for encoding and some other stream handling. However,it seems that FFmpeg GPU HW-Acceleration is not supported on jetson tx2,but I can use omxh265enc plugin in Gstreamer for GPU HW-Acceleration purpose. So, I want to konw how to call this Gstreamer encoding plugin within FFMpeg.
Can you give a detailed description for me? Surgent!

It is not supported in default releases. We are evaluating to support it in future release.
A community contribution is shared. Please check