This plug-in will create guides every X pixels apart for the entire image size. It can handle both vertical and horizontal guides.
I wrote this for easier management of multiple textures and sprites in a single image file for graphical programming.
Copy the file in to your Gimp scripts directory, usually '~/.gimp-2.x/scripts'. Refresh the scripts from Gimp menu 'Filters'->'Script-Fu'->'Refresh Scripts'. This is unnecessary if Gimp is being re-started. Once this is done the script can be accessed from 'Image'->'Guides'->'New Guides Every X Pixels'.