Apply gradient to part of text

You need your text layer on the bottom and a gradient layer that just covers the parts of the text you want to fill with the gradient, then apply Set Matte to the Gradient layer and choose the text layer as the source. This is everything that is required.

This is only 1 of about 10 ways I can think of to part of a layer with a gradient. You could also apply a mask to the text layer and add Gradient Fill (not nearly as flexible as a shape layer which can have as many gradients applied and blended as you like) or 4-Color Gradient, then use compositing options to apply the effect only to the text inside the mask. Again, I'm showing you everhting that you need to recreate the effect.