I don't believe there are any pre-existing GStreamer plugins that are capable of doing fall detection. If you're sure that you want to use GStreamer as your machine learning framework of sorts, you may be interested in Nvidia's DeepStream. It's a system made of GStreamer elements that run machine learning algorithms on videos. It's free as in beer and partially open source. It's an impressive system, but requires quite a good understanding of GStreamer internals in order to fit it to your use case, and you'll still need to find or bring your own fall detection model.
I work on a product, called BrainFrame, that uses GStreamer for video streaming but uses a different, more easy-to-use system for the machine learning portion. I don't want to do too much self promotion on the mailing list so that's all I'll say about it here but feel free to reach out if you're interested.