Нашёл в гугле ешё вот что: "The .goutputstream files are temporary files creating while copying or appending a file. They're supposed to be removed automatically but I seem to recall some GIO bug awhile back where they were getting left behind, perhaps if the operation was interrupted or something. In any case they too are safe to delete".
Тоже добавил плюсик к этому багу.