Make the image any size you want, preferably not huge though.

Using the gradient tool, in options where you choose your gradients, there's a drop-down bar that should say 'Normal'. Change that do 'Difference'. Also make 'Linear' to 'Radial'. Now select a gradient of your choice.

Now with the Gradient tool at hand, drag from the top left down to the bottom right. Now do bottom right to top left. Play around with it, doing it from wherever you want, as long as you want, etc etc. The more colors you have in the gradient the more wackier and crazier it gets. Mine turned out like this.