Grid lines, cut out the letter, turned the letter, gaussian blur with the chains unhooked for the letter lines. Once I had it the way I wanted it to look, I turned the whole thing. It's really just grid, dodge and burn layer modes. Needs tweaking and I could probably have done better with a larger resolution. But nonetheless, here's my try.

[ Image ]

With four levels:

1. copy image, convert to B&W
2. Filter/Blur/Pixelize, with 1px horizontal and 20px vertical
3. While this image is displayed, go to the Channels list and copy one of the RGB channels
4. Using filter/render/grid, generate N layers of horizontal lines on opaque background with:
   
   • Spacing = 20 (the pattern width)
   • Offset=10 (half the pattern width)
   • Thickess=4/8/12/16pixels (visually, 20%, 40%,60%,80% of black on white)
5. Make sure these layers are ordered with the thinner lines on top.
6. Add a layer mask to all layers except the bottom one, initialize to the channel copy you did
7. Threshold each of these masks. The 4px (20%) layer will show for everything from 30% , so it's threshold is around 255*.7= 178, the 8px (40%) layer represent everything between 50% black and 30%, so the threshold is 128, and likewise the 12px (60%) layer is around 255*.3=76.

i cant quite replicate your Portman meets pop-art image, and i dont know why
did you process the grayscale layer some more, or did you just apply desaturate on it ?
(attached the xcf)

[ Image ]

Your bottom layer (16px) should have no mask (or a mask at full opacity). Bucket-fill the mask with white or remove it.