Is there any case to show saving video to SSD using the PCIE 4 X8?
If the storage is mounted, you should be able to save to the disk by modifying path in filesink.
$ gst-launch-1.0 videotestsrc num-buffers=300 ! nvvidconv ! nvv4l2h264enc ! h264parse ! qtmux ! filesink location=/home/nvidia/a.mp4
Change /home/nvidia/a.mp4 to desired path. Please give it a try.