This might depend on your OS/distribution and GStreamer version.
Over here (Debian jessie, GStreamer 0.10.36) gst-inspect ffdec_h264
gives the following output:
Factory Details:
Long name: FFmpeg H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 decoder
Class: Codec/Decoder/Video
Description: FFmpeg h264 decoder
Author(s): Wim Taymans <wim.taymans@gmail.com>, Ronald Bultje <rbultje@ronald.bitfreak.net>, Edwar$
Rank: primary (256)
Plugin Details:
Name: ffmpeg
Description: All FFmpeg codecs and formats (system install)
Filename: /usr/lib/x86_64-linux-gnu/gstreamer-0.10/libgstffmpeg.so
Version: 0.10.13
License: GPL
Source module: gst-ffmpeg
Binary package: FFmpeg
Origin URL: http://ffmpeg.org/
So on my system, ffdec_h264
is in the gst-ffmpeg
module (which was installed using apt-get install gstreamer0.10-ffmpeg
).