Well... one way to go about this is to just overlay a gradient on the text. My recommendation would be to create a Rectangle shape and use the Fill Mode set to Gradient. Create the gradient you need to fill the rectangle. This is not as easy as it is with text Face gradients. You will need to set the Start X and End X parameters so that the gradient spreads out evenly across the Rectangle. Set the Start Y and End Y parameters to 0.
Fitting the Rectangle to the text:
Go into the Rectangle > Shape > Geometry tab and right click on Size > Add Parameter Behavior > Link. In the Link Source Object well, drag the Text object and drop it in there. For the Source Parameter, click on Compatible Parameters and select > Object Attributes > Size > All. This will make the Rectangle shape the same size as the Text object. Next, go to Behaviors > Basic Motion and select Align To. In the Align To Object well, drag the Text and drop it in. The Rectangle should align exactly and cover the Text object.
Now, for one of the slickest tricks in all of Motion:
Make sure the Rectangle is **over** the text (higher in the layers list) and in the same Group. Go to the Properties tab of the inspector, and down in the Blending section, click on the checkbox option to Preserve Opacity. (Stunning, isn't it?)
OK - that's one way.
Here's another a little more simple, but not quite as obvious.
Convert your Text to 3D Text. In the Substance section, change the Substance to Generic. For the Surface, select Gradient. Design your gradient.
To make the text appear 2D, under 3D Text at the top of the inspector, set the Depth to 0 and the Front Edge Size to 0 (or set the Front Edge to Square).
There is a difference between the two methods. The 2D Preserve Opacity method will leave a "tell-tale" (very thin) outline on some of the characters (this is due to anti-aliasing to help smooth the appearance of text edges). The 3D version will not display the anti-aliasing outlining.