I've been doing some tests with Gstreamer and what seems to be happening is that your command is not being recognized because the plugins are not installed. You need to find all the plugins that you are trying to use in that command like omxh264enc, h264parse, and all the others and install them.
In case of v4l2src, you have to install the plugin called video4linux2, for tcpserversink you need the plugin tcp, both of those can be found in AlexT's repo and can be installed with the commands:
opkg install gstreamer1.0-plugins-good-video4linux2
opkg install gstreamer1.0-plugins-base-tcp
Most of these plugins are in plugins-good, and plugins-base. There are others like plugins that are not on AlexT's repo.
You can use gst-inspect-1.0 to see which plugins are available to use in Gstreamer. You will have to look for all the missing plugins and install them manually.