If you want to use a USB camera (these types of cameras are also called 'Web Cameras') with GStreamer on i.MX6 devices (Linux Kernel version >= 3.035), you need to either load the module dynamically or compile and link statically selecting (Y) the following config on the Kernel configuration
Device Drivers -> Multimedia support -> Video capture adapters -> V4L USB devices -> <*> USB Video Class (UVC)
After the Kernel image has been built, flash it into the target, plug the web cam, then on a (target) terminal run
gst-launch v4l2src ! mfw_v4lsink
You should see what the camera is capturing on the display. In case you need to encode the camera src data, you need to place the encoder into the pipeline
gst-launch v4l2src num-buffers=100 ! queue ! vpuenc codec=0 ! matroskamux ! filesink location=output.mkv sync=false
We are using a certain codec (codec=0 means mpeg4), check options using 'gst-inspect vpuenc'.