Definitely a radial gradient as iDad said, which you will find in layer fx. White in the center blend out to your darker set color. Just set the colors you want in the foreground/background.
Another way to make the gradient besides using the layer fx, is to do it "manually" on a layer above and clipped to the letters. If it helps you to see the entire gradient initially, instead of just through the letters, you may prefer this method. I do; just my preference.
Choose the gradient tool, right click on the image of the gradient. Set your pointers at the bottom to the color you want and move the little blank diamond stops to create the spacing/blending of the colors. Then just draw your linear gradient across the letters. If you use radial, you should be able to get the desired effect with one color at one end and one at the other. In other words, you probably won't need to do the right clicking, etc. Just trying to trick you into learning more about the process!
Here's one interpretation for you.
Hmmm, if I misinterpreted what you want to do, and it is for one letter at a time, then basically apply the same method as iDad shows. You'll just need one layer for each letter to apply the layer fx individually. Make your word, rasterize, then make new layer from each letter. That way you won't have to play with spacing.
. . . Or did you mean the background? If so, you do it essentially the same way and radial is the best choice.
It may also be, in the example you posted, that more than one gradient layer is used, and one is set in a different blending mode. Those kind of things simply require experimentation. The may even be layer mask to soften some edges and make the shape more elliptical.
Nice to meet you everyone. I am from Bulgaria, so sorry for my bad english 
