Glad to see that I’m not the only one that is still playing with this one 
I gave my old love, Kodak T-MAX 3200 a try.
This was done with GIMP.
I still think that the grain in digital is to even (spread and granules). If anyone knows how to create more realistic (more uneven) looking grain please enlighten me!
Gave it another go myself because it is really fun to be honest. I spend a lot of time trying different combinations of stuff to shift things. Though I have not tried to emulate a particular film directly. This one I have done I tried to make it a bit less warm but also bring magenta into the shadows similar to the one Kodak film I can’t remember the name of.
Yes grain in digital is a bit too even. I used the grain module in dt. The only grain I have seen that tends to bring the film like grains to digital without the uniformity issues is Capture One and the DxO Film Pack. This is because they are maping the grain to the image so grain only ends up where it should and the grain maps are from actual film. dt does a good job as well mapping the grain but it seems to only have one pattern.
I do think I have a way to do it but it will take some time to create it: One could create a image that contains random noise (think old TV noise/static). This is inherently random and does vary in size. It does need to be fairly big in size (for me it should be able to easily cover a D750 RAW (FX roughly 6100X4100).
This image, as a new layer and after using some filters/effects, can be merged (grain merge?) with the below layer. Works in theory…
could put the grain on a 50% gray layer then use Overlay + Luminosity mask. Still a pain. I always found grain a bit easier with something like Photoshop if doing it manually because of blend if allowing you to mask it into the realistic areas quickly. This method can also be used with film grain overlays that were taken with film cameras against 50% grey cards that you can download off the internet.
Never used, or played with, Lightroom and/or Photoshop and am thus not sure if GIMP is up to par. That is if comparing them is wise to begin with, one tends to know more about one over the other.
(Layer) Mask and blend modes will definitely be needed to get a good result. I really need to give my (and your) idea a try. So little time, so many potentially nice things to do 
IMO, the one major area where Photoshop outstrips GIMP is preparation for printing. And GIMP may have caught up some since I dug into the comparison.
One thing Gimp can’t do is underlying layer blend if. Does not really matter as a luminosity mask can do the same thing if if you target it to the correct tones and use levels to tweak. Not having adjustment layers is bleh but it doesn’t stop you from doing the same kind of edits.
I’m hoping a new module or plugin or something comes of this great work one day, but in the mean time my preferred method is using the Add Grain filter in G’MIC. I quite like the TMAX 400 option, and find the Overlay mode to be quite a bit more natural than the Grain Merge default.
I remember reading a paper about Phase One experimenting a Filmulator-like approach, literally simulating grains and their growth, to mimic the effect of real film grain.
I wonder if I could hack something similar into Filmulator…
yea their film grain module is pretty darn good it even auto scales the grain when the image size changes via crop or export to ensure it is the same across all resolutions possible. It also targets the appropriate places where the grain should be instead of across the whole image.
Capture One in general is a really good converter imho just so expensive I am trying to replace it lol.
That is nice, isn’t it! Would indeed be great to see that implemented in either G’MIC or darktable.
I did have a look at the G’MIC TMAX option, but didn’t use it due to the no LUT/CLUT rule for this challenge. I do realize that it isn’t a (C)LUT, but I did feel like using it was too close to cheating 
@CarVac / @blj : I hear great things about Capture One. Maybe I need to buy myself a Phase One IQ4 system so I get Capture One for free 
If only I could afford a $50k camera setup so I can get $300 software for free. I mean it is not that I can’t afford the software it just seems like the $20/month could be better spent else where on other hobbies lol. It is really good software though.
G’MIC TMAX would be fine it is just a grain filter not a LUT/CLUT. The no LUT/CLUT rule was more to force the concept of a manual color grade for more creativity then just using a pre canned film grade.
A quick a easy idea. I decided to stimulate film reticulation. Took me only 3 minutes. Didn’t do other modification as I wanted to try stimulating reticulation only.
Done in Krita with G’MIC Perlin Noise on YCbCr channel, then blended in with P-Norm A Blending Mode. I did do cross channel adjustment filter mask on a clone of original layer and set that to Destination In. I filtered the dark target part of the Perlin out.
Can’t help but see a grid-like pattern, which makes it less believable. Once you work out the kinks, I think it would look nice on High ISO Edit Challenge.
Now that you mention it, yeah after seeing it, it doesn’t look all that real because of a grid-like pattern. Maybe a new filter after theorizing how to make perlin noise more in the fashion of actual film reticulation.
Here are my versions using darktable. In general I used:
.
