I just need to do the same process that was done in
but in GIMP.
I can add noise to the image in gimp, but I don’t know which sliders should I move to get the wanted effect.
There’s an easier way
Create a new image with as many pixels as you need.
Choose a new foreground colour, i.e. a green colour for the grass, and fill the canvas with it using Edit > Fill with FG colour
Do Filters > Noise > HSV Noise, and move the Value slider until you get the effect you want.