Oh. I just looked at the tutorial link you posted, so I misunderstood exactly what you were asking.
...Well, you could have one of your colors set to transparent and the other white...Then make a new layer and add your linear white gradient and use the selection to cut it off where you want, or you could draw an opaque, white shape and use the transparency gradient as mentioned in the tutorial.
Your gloss shape should be on a new layer.