Thanks to the newly added atenc element, you can now use Apple's well-known AAC encoder directly in GStreamer!
gst-launch-1.0 -e audiotestsrc ! audio/x-raw,channels=2,rate=48000 ! atenc ! mp4mux ! filesink location=output.m4aIt supports all the usual rate control modes (CBR/LTA/VBR/CVBR), as well as settings relevant for each of them (target bitrate for CBR, perceived quality for VBR).
For now you can encode AAC-LC with up to 7.1 channels. Support for more AAC profiles and different output formats will be added in the future.
If you need decoding too, atdec is there to help and has supported AAC alongside a few other formats for a long time now.