Cannot Capture Video Stream Using External USB Camera on OpenCV

This is wrong. gst-launch-1.0 is a binary command for launching a gstreamer pipeline from command shell.
For opencv VideoCapture, you would just provide the pipeline string ending into CPU memory appsink with BGR format for color. Try:

const char* gst = “v4l2src device=/dev/video1 ! image/jpeg,width=1280,height=720, framerate=30/1 ! jpegparse ! nvv4l2decoder mjpeg=1 ! nvvidconv ! video/x-raw,format=BGRx ! videoconvert ! video/x-raw,format=BGR ! appsink drop=1 ";