Hi,
Generally we call gst_parse_launch() to initialize the pipeline. There are C samples in the posts:
Camera liveview cannot show up again after repeated open/close the liveview for several times - #23 by DaneLLL
Getting Error for nvv4l2h264enc in application compilation - #7 by DaneLLL
GStreamer freeze when using qtmux and NVIDIA-accelerated h264/h265 encoding - #7 by DaneLLL
Please check the samples and try gst_parse_launch()