Chris,
You would either need to add a whole lot of gradient stops, changing from white to the yellow, or simply convert the text to outlines and create a gradient swatch and apply it to each letter.
But that design isn't simply a gradient. There is also a clipped blend of lines/rectangles that also have a gradient in them running from white at the top to yellow at the bottom. I would build this logo in the vector drawing application of choice.
Here is a quickie version, different font. Gradient colors are a bit off. But note the lines in the upper bitmap from your post.
