It is used to build a pipeline processing audio/video/metadata. You can use some of ffmpeg plugins with it but it is a separate product.

GStreamer is a library for constructing graphs of media-handling components. The applications it supports range from simple Ogg/Vorbis playback, audio/video streaming to complex audio (mixing) and video (non-linear editing) processing.

It’s more of a glue that can connect multiple media things together