What exactly? "Gradient Editor" and "Color Picker" are two completely separate things. A color picker can be used to also pick colors in a gradient amomng other things, but otehrwise the only dedicated gradient editors you will find in AE are the ones associated with shape layer gradient fills and strokes and the Gradient Overlay layer style. All other gradients are doen with effects. If you are referring to the scratchboard7 mixer area, then this would be dependnt on what color picker you use. In the Adobe color picker, it will adapt to which color channel is currently active, in the Windows color picker it may simply be collapsed and can be expanded by hitting the more button...
As Mylenium suggested, you only see the gradient editor when editing gradients in two contexts:
"If you click a gradient swatch for a stroke or fill in a shape layer, or click Edit Gradient in the Timeline panel, the Adobe Color Picker opens as the Gradient Editor, with additional controls for editing gradients included at the top of the dialog box."