IPTC is the industry standard for photographic metadata so that is what you should use. As usual, a little shell scripting makes life easier so let me introduce you to the comprehensive answer to all your EXIF and IPTC metadata manipulation needs : Exiv2. Using it I whipped up a trivial, quick and dirty way to tag a bunch of files with the generic IPTC metadata of your choice. Edit the script to your taste with your own data and you are set.

Yes it is most trivial, but it is news to me and it is all I need… So there – scratching your own itch and all that…

Last but not the least : be aware that Gimp will rudely overwrite IPTC metadata with no warning and no point. The developers are aware of that and fixes will be forthcoming in an undefined future release. So along your digital photography workflow make sure that you tag your batch of pictures downstream from editing them in Gimp…