Tag writing is done using the GstTagSetter interface. All that's required is a tag-set-supporting element in your pipeline. In order to see if any of the elements in your pipeline supports tag writing, you can use the function gst_bin_iterate_all_by_interface (pipeline, GST_TYPE_TAG_SETTER). On the resulting element, usually an encoder or muxer, you can use gst_tag_setter_merge () (with a taglist) or gst_tag_setter_add () (with individual tags) to set tags on it.

A nice extra feature in GStreamer tag support is that tags are preserved in pipelines. This means that if you transcode one file containing tags into another media type, and that new media type supports tags too, then the tags will be handled as part of the data stream and be merged into the newly written media file, too.