try stroking a selection with a brush
endless fun !
N ot an animation but stroking a path combined with brush dynamics can come with some interesting shapes. Much improved since Gimp 2.6
This is using a 'comb' type brush, could not be much simpler.
Looking at the brush dynamics, again the simplest of options , only colour and fade
but then try adjusting some parameters, obviously the fade length, also brush angle, the repeat option and the gradient. Also try a containing selection and also the selection with a large 'feather' value.
As Esper said great fun