I've made a version of the GIMP script to support some of the features of my Photoshop script.

https://gist.github.com/flyover/e6e1f125e5aef585815f

It uses a GroupLayer for organizing bones, slots, skins and attachments of a Spine project. The exporter uses the Layer name to connect attachments to slots and slots to bones. It also uses the center of the bone layers for the bone location. There is also basic support for importing a JSON file back in to GIMP (regions without any rotation).

Here is the Photoshop project:

https://github.com/flyover/psd2scml