It's a lot easier to do than it looks!
I usually do this with glosses, because they mix together better and create more of a blended effect.
With this I have not blended the colours in with one another as much as I normally would because I wanted the technique to be really clear.
I have used two of my favourite glosses here:
For a red/gold look like the one I am sporting here, first outline the lips with the red gloss, leaving the centre of your lips bare.
Then fill in the centre with the gold gloss.
After that it is just a matter of carefully blending - adding more of the lighter colour (in this case gold) to blend it in is the way to go.
Be careful how you blot your lips, if you usually press your lips together, be careful, or you will mess up your carefully applied gradient.
That's all there is to it, really - have fun with it and share your gradient adventures in the comments.