This can only be done if you switch your file to RGB ( Files / document color mode / RGB ). Then double click on the color pin in the gradient tool and Voila. you got your RGB. You just have to do it on each Pin because it is not certain that the color tool will switch ( though it is supposed to ). So double click the pin just to be safe.
Erratum : For the color tools, you have to click its option ( top right corner of the tool ) top switch to RGB.
You still can double click the Pin in the gradient tools.