Gimp generated "paper marbling"

[Espermaschine]

I would make this the same way its done in the real world:
drop colour on the canvas, then comb and stir.

So draw random dots and shapes on a canvas, then have fun with the Warp Transform Tool. (aka iWarp in previous Gimp versions)

   

Here i added a bit of Ripple filtering and G'MIC Array [faded].