GStreamer is not "just a different front end to ffmpeg". GStreamer is a pluggable media graph system.

"GStreamer" doesn't link against libav. The GStreamer plugin "gst-libav" links against libav. If you're not using the gst-libav, you're not using ffmpeg. I'd bet a relatively small amount of GStreamer use cases use gst-libav; I typically see people use e.g x264dec and x264enc (from the x264 plugin) to decode and encode media, or, for hardware encoding/decoding, the v4l2enc and v4l2dec elements (or elements from a SoC vendor's plug-in such as gst-rockchip). It also has its own non-libav elements to handle container formats, pixel format conversion, scaling, etc, which are more natural to use since they're part of the core gstreamer plug-in packages rather than gst-libav.