On second thought, that one uses a filter that's not in the gimp anymore.
Here's what I'd do:
Filters > Render > Clouds > Solid Noise
Set the x and y size up high
Filters > Noise > Spread
Set the spread amount up quite high (like 80?)
Filters > Map > Make Seamless
Then do either:
(a) Filter > Blur > Gaussian Blur
with different horizontal and vertical values