[How to make this effect]

Note: set the quantization mode to Octree, otherwise you will get this mess:

[How to make this effect]

i finally found a definite solution! i found this in PyroChild's plugin pack, but you may be able to find it in other places. 
Here, i have generated a gradient:

Then, i use this handy effect called Quantization

(Location as of PyroChild's plugin pack)

Here, set the dithering level to maximum and set the colors to anything of your liking (average value in GD is 8-16 depending on the texture size)
I noticed that the gradient i am using was quite exaggerated, and it would work better on more subtle and graduate gradients such as the ones in GD

hope i helped, i have been searching for a solution for a very long time to be honest