[facebookvideo][/facebookvideo]I just looked in GIMP/Help/Procedure Browser and see:
they look simple enough to use.
Although, I don't know what the metadata string looks like I would try manually editing the metadata
and then use gimp-image-get-metadata to get the metadata string, then examine what the format looks like
then once you know what the format looks like you can easily build your own string with values you want to set and then use gimp-image-set-metadata to set it. and test it then use the front end to see if it properly loads the metadata for you to edit and such.