Hi,
You can turn the grid on and off by the menubar.
Click View --> Grid to turn on and then do the same to turn off.
You can customize the grid by RIGHT clicking on the image window. In the pop-up menu list choose Change Grid, Guide and Snap Properties. This will open the dialog box. In the dialog box you can customize the grid for the current image you are working on and also you can customize the default setting that will be used when you turn on the grid.
Horizontal and Vertical grid -- you can change the distance between grid lines and you can change the units to pixels, inches or centimeters.
Color -- you can change the color of the grid lines
Snap influence -- you can change the sensitivity of the snap feature, that is, how close an object must be to the grid to snap into place