You need to make a greyscale image and export it to a gbr brush.
White is transparent, black is opaque, greys are semi-transparent. Greyscale will take the brush foreground colour.
You can use the image of the hand, or paint your own. I used the image
Colours -> Threshold to get it mainly black hand on white. Colours -> Invert to get a white hand. Image -> Scale to get down to brush size.
Make a greyscale image (brush size), white canvas and fill with black noise. Noise -> CIE lch
Copy / paste the hand over the noise, put in lighten-only mode to give white hand in noise.
Layer -> New from Visible, paint around the edges in white (remember white is transparent in the brush) to fade out.
Export to your brushes folder ~/.config/GIMP/2.10/Brushes
There will be other ways, but all that as a video demo:
https://youtu.be/s7uFOGoEnek 4 minutes
Amazing. Above and beyond anything I expected, thank you!