How do I edit a JPEG's EXIF data with Gimp?
As of Gimp 2.9.4 (possibly 2.9.2) there is a built-in metadata editor/viewer. It is under the menu > Image > Image Metadata.
See http://libregraphicsworld.org/blog/entry/gimp-gets-advanced-exif-xmp-iptc-metadata-support
If you are on OSX and want a binary of this version (rather than compile from source), try Partha's McGimp variants, the McGimp 2.9.5 64-bit Experimental, or McGimp 2.9.5 64-bit Color Corrected Experimental, an interesting project in its own right.