From what I understand gstreamer is more about building complex pipelines and plugins, ffmpeg is better at playing some obscure 20 year old video format extremely efficiently so you can watch it compiled for a potato.

Different cases really I think both are good.

That's not really true. Ffmpeg is a Swiss army knife for anything related to digital multimedia (old and new). It is broken into a few libraries but doesn't really have plugins.

Gstreamer has a different model, chaining together plugins. Lots of overlap, but I think Gstreamer only has real traction because some silicon vendors use it.