KDE

Yeah. The ffplay documentation says:

FFplay is a very simple and portable media player using the FFmpeg libraries and the SDL library. It is mostly used as a testbed for the various FFmpeg APIs.

Given that, I would expect that it actually doesn't make use of GStreamer, just to really keep things as simple as possible.

From a programming perspective, it isn't really surprising that these projects have overlap. You see that quite often, that some venerable library, like ffmpeg, does a massive chunk of the grunt work and then you've got libraries like GStreamer and libmpv, which sit on top of that, and 'just' integrate it into a wider framework or tie it all together for a specific purpose.

From an outside perspective, that will make it look like they're all similar, because the core of the magic, ffmpeg, is included into all of them (I assume).
It's just more confusing here than in other projects, because each of these projects is visible to us users in some way.