Can G'MIC do those kind of textures?

[David_Tschumperle (G'MIC staff)]

Just an intuition, needs probably more work:

foo : skip ${1=50},${2=150}

  # Create random gradient.
  $1,1,1,5,"[ u(500),u(500), u(255),u(255),u(255) ]"
  rbf. 500,500 n. 0,255 equalize. 256

  # Create random deformation.
  $2,1,1,4,"[ u(500),u(500),u(500),u(500) ]"
  rbf. 500,500
  warp.. .
  rm.

Then, try several values for first and second arguments.