Gradients can be applied to both text and shapes. Most of the clipart is either multi-color vector or bitmap based, so gradients can not be applied to those elements.
To add gradients to text or shapes, simply select the element either by directly clicking on it in the canvas, or be selecting it's layer in the Design Layers palette.
To remove a color picker, just click the black triangle above a color picker and then click the Remove Color button.
You can adjust the angle of the gradient with the Angle Slider.
You can also choose Radial for adding circular gradients.