Very easy trick on how to make a complicated/multicoloured gradient in GIMP. Use the pallete dock and make a gradient from there. This GIMP tutorial shows you exactly how to do it.
This Gimp how to shows you how to
open a dock in GIMP
make a gradient in GIMP
make a pallete in GIMP
make a gradient from a pallete in GIMP
Link to how to install Python on windows.