gridlines don't print - you need to add "borders" if you want to see them
when you print, to do this, select your table (table / select table) and
choose format / borders and shading - click ALL or whatever you want and
then check out the result in print preview.
To clarify, gridlines are only on screen lines showing the edges of cells in
tables or Text Boxes, to assist with editing. Printable 'gridlines' are
Borders as Julie has described.