Here is a very easy way I've made grid lines for a map as a new layer.

First. Open up a spreadsheet app like Excel or Calc. Set the column width and row height to match each other; ie 1inch x 1inch.

Take a screenshot.

Paste that as a new layer. Use the magic wand to get rid of the white space and then get rid of the rest of your screenshot leaving behind a grid.

Hope that helps!