Here's the Mercator graticule again with the background turned off. This way you can simply load it straight into the GIMP.
And here's a template for the ever popular dual hemisphere map with polar insets. It's in stereographic projection with a 15° graticule and the insets cover out to 60° N/S. Like the above one, you can open it directly in The GIMP to get a transparent backed graticule, or in Inkscape where you can unhide a background layer and feature layer masked to fit in the template for your convenience.
You can use them under Creative Commons - Attribution. http://creativecommons.org/licenses/by/3.0/
Here are some quick example images