If you wait another week you'll be excited about the kinds of effects you can apply easily
And normally you want to make gray some area outside a specific selection, which you would do by selecting a region and then choosing SpecialFx menu -> grey outside (or choosing the button to do this).
BUT, if you really want to grayscale the entire image, you can choose SpecialFx -> Adjust Colors
then go to HSV (or HSL) tab and slide the Saturation slider all the way to the left.