First, I have no idea of how to do it in Gimp. Second, in PS this is a tremendously tedious thing to do...lots of undo and try again involved. The most success that I've ever had in doing this was not with filters or anything automated. Instead I just use the eyedropper to choose a color then hand paint over the area in question...blurring, smudging, distorting along the way. Copying and pasting small sections into place also works with some hand tweaking. PS clone stamp and pattern stamp tools also help a bit...not sure what Gimp has. In the end it always involves lots of hand tweaking and hair pulling.