Freeform Gradient Tool
Here's some novice tips while working with Freeform Gradients:
It's better to use a light background.
The preferable blending mode for objects is Multiply.
Define from start the color palette in which you plan to draw.
Add a noise effect at the end (Effect - Texture - Grain).
I hope these simple tips will help you to better understand the tool, and don't be afraid to experiment, it sometimes brings very unexpected (and much interesting) results.
To better understand and figure out how it works, download o the asset images of this tutorial, they're are free to explore:
Have fun with making your own cool gradients!