You have two options for making a gray background on a business card: Print it on gray paper stock, or print the gray color onto white paper stock. You choice is going to depend on what else is on the card (you might not want to print colors over gray, for example).
To SIMULATE gray (or any other color) paper you can edit the [Paper] swatch. this is purely an on-screen aid and has no effect on output -- no ink will be used anywhere there is [Paper] color. THis will not be particularly accurate as a preview since it will not really reflect the interaction of other colors on top of the gray paper, but it's better than nothing if you need it.
To PRINT gray you need to add a frame to the background and fill it with the gray of your choice.