Open up the image you want to put the gridlines on. In this instance I am going to use the image on the bottom.

Now open up a new image that is 10x10 and background color is transparent. This will be used to make the grid later on.

With that image open zoom in all the way. Take the rectangular selection tool(Antialias: 0 | Feather: 0) and select 2 pixels across the top and 2 pixels down the left. Fill in the selection with the color you want the grid to be.

Switch back to the image you want the gridlines to be on. Take the airbrush tool(Shape: Square | Size: 60 | Hardness: 50 | Opacity: 7 | Step: 25 | Density: 100) feel free to play around with some of these settings.

Now go to the foreground color. You have to change the option to the foreground pattern instead of the foreground color. You do this by clicking the small black arrow on the foreground color box.

Change the pattern to the image you made previously which was the 10x10 image. For the scale make it 50% and for the angle make it 0, feel free to play around with these settings.

You now have to add a new raster layer(Layers-->New Raster Layer). Name the new layer "Gridlines" with these settings on the bottom.

Here comes the fun part where you add the gridlines! With your airbrush tool selected airbrush the gridlines onto the image. Be creative with this part, make some parts lighter than some and some darker than other parts.

Lets say you have an image like this and you added gridlines on it. Obviously it looks awful because the gridlines are right on top of the animal. The next step will tell you how to get rid of the lines.

First off select the eraser tool(Shape: Round | Size: 23 | Hardness: 0 | Opacity: 50 | Step: 1 | Density: 100 | Buildup: Unchecked), feel free to play around with these settings. Zoom in so you can see where you don't want the gridlines and using the eraser tool slowly erase the gridlines that you don't want.