From the Swatches panel's flyout menu choose New Gradient and click on the end points of the Gradient Ramp to choose the colors. Give the gradient a name and apply it to a frame as a fill color.
Make sure you have two SPOT swatches available, or else your gradient will contain 4 process colors. If you only want the two colors in the file, make sure you choose those colors in the gradient panel (in pull-down menu, choose swatches). Then your gradient will use only those two spot colors. Hope this helps.