Hi,
Please try gst_parse_launch() like this sample:
Nvidia-desktop kernel: [407343.357549] (NULL device *): nvhost_channelctl: invalid cmd 0x80685600 - #35 by DaneLLL
It shall work well and no need to add code bout pad_added_handler()
For more gstreamer samples, please also check
/opt/nvidia/deepstream/deepstream-6.2/sources/apps/sample_apps/