Are you sure of that? becuase while programmes may lable the file as a jpg file when they open it they are working on the full sized file, less of course the material the camera has thrown away ... not ever met GIMP but it is most unusual and I doubt if any serious photograhers would use it and there are quite a few at this site doing so. I hope they clear the matter up for you.
If you can set your computer to do it, jpg can be almost lossless ... it has gotten a bad name from people compressing their files when saving as a jpg without realising what they are doing.