Here is a really easy How To 2 Color gradient tutorial I found on YouTube. It is by Jess Bunty.
I know there are a ton of tutorials out there that look much nicer in the end. But I liked and chose this one to post because it is short (only 2:23 minutes) and clear to the point on the basics of how to do a gradient.
The only thing I would add is that I dab my sponge on a piece of paper towel before I apply the first dab to my nails. This gets off the excess polish that hasn't seeped into the sponge. It seems to let me be able to do thinner, lighter coats so it doesn't appear so "goopy" on your finished nails.
I also love to put a glitter top coat on my gradients... It hides some of the imperfections