decode a mp4 video with gstreamer

I don't think there's a x264dec element (I only know of the x264enc element). You might want to use ffdec_h264 like in this nabble post:

gst-launch-0.10 filesrc location=$1 ! h264parse ! ffdec_h264 ! ffmpegcolorspace ! deinterlace ! xvimagesink

This might be easier:

gst-launch-0.10 playbin2 uri=file:///path/to/foo.mp4

or

gst-launch-1.0 playbin uri=file:///path/to/foo.mp4

(You still need the required elements installed of course).