Gradients are one of my very favorite nail art looks: when done well, they're so impressive! I'm still trying to perfect my own technique (fully blending the colors is my biggest challenge), but I'm reasonably happy with how my first three-color gradient turned out.
I started with a base of Essie Rock the Boat, and then once that was dry, painted Rock the Boat, butter LONDON Slapper, and Essie Butler Please onto a makeup sponge and dabbed it several times onto each nail, adding more polish to the sponge as needed. (I also taped off my cuticles before doing the sponging to save myself a lengthy cleanup.) Finally, I finished everything off with two coats of nails inc. Kensington Caviar.
As you can see in the pictures, I can definitely do better blending the colors (particularly on my thumb)—I think I need to focus more on moving the sponge slightly as I dab it onto the nail. I also need to try to get good coverage with fewer layers: this ended up being quite a lot of paint, and as a result, it chipped way too easily!