Open the Inspector on the icon bar at the top right of your Numbers window.
Click on the "Cells" icon on the Inspector toolbar. This will bring up a number of options for setting the view ability of your gridlines.
By default, Numbers uses a "thin" gray line for viewing while working but that line disappears when printing. To make the line visible in the final print job, select a line color by clicking on the color picker box.
If you need a thicker line, you will need to change the line selection from "thin" to one of the other line options (normal, dashed, dotted).
This will activate the thickness window which will allow you to set a thicker line.
Simply click on the selection arrows and increase or decrease the line thickness. You can also type directly into the line thickness window.
HOWEVER...there is a better way to do this if what you want is to make your spreadsheet more readable. Here's how.