For Gimp, there's a cool script available that is called Shellout: http://registry.gimp.org/node/24977
This python script will call an external program, providing the active layer or a new from visible layer to this external program via a temp file. Once the external program is completed, it should save/overwrite the original temp file which gets pulled back into gimp.
This is extremely useful to use standalone versions of plugins that don't work correctly via the Photoshop Plugin Import in Affinity Photo, for example, some plugins of the Nik Collection.
It would be really useful to have such a "Send to standalone and Back after editing"-function, especially if you have some programs that aren't available as plugins but want to keep your workflow efficient.