I use darktable for every photo - it’s my RAW-converter and I do everything about color, white-balance, b/w conversion, contrasts and the like in dt.
The Gimp is as important, but I use it only on about half my pictures (or less). Dodge & burn, get rid of that electric-lines in the background or make a texture just that bit more smooth is what I do with the Gimp. But not every photo needs that kind of treatment, so I don’t use the program on every picture.