>>2886589lol, presets.
wb off empty film
crop in so it's just image (no border, no sprockets, this is important for your histogram, youc an change it back later)
min higlights, max shadows, set your white and black point using alt.
invert RGB curve
Top and tail the r, g and b curves so the histogram peaks and troughs line up
congratulations, your image is now adequate and comparable to a raw out of camera
If there's anything there you don't understand, go learn lightroom some more, gimp is a waste of everyone's time - no-one is going to advise on the intricacies of shit software, by using shit software you acknowledge that you can read the instructions for popular similar software and "translate" them into your product.