GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring. The terms of usage and rules about copying are clearly listed in the GNU General Public License. There is a nice Frequently Asked Questions (FAQ) page.
There are four plugins for 360° equirectangular panorama creation - fill_zenith_nadir, panorama_to_zenith_nadir, zenith_nadir_to_panorama, set_google_plus_metadata - Linux (Ubuntu 12.10 AMD64) | Windows (Thanks to @Treer for pointing out the GIMP plugins and building the Windows versions) - April 2018 UPDATE: From Tom Jones on Facebook: “From Panorama Projection Filter now has reverse transform in the latest test version of GIMP. If you find any bugs with it feel free to report them as they are very responsive. I asked for the reverse transform and they added it in a day or two =) Should allow easier editing of 360 panoramas in GIMP similar to PS. Final version should be out quite soon so get those bug reports or feature enchancements in.” https://www.gimp.org/news/2018/04/17/gimp-2-10-0-rc2-released/