This script quickly desaturates all layers in an image using the chosen method: Lightness, Luminosity, or Average. You can download the script here:FP's Desaturate All Layers Script
The preview image shows the left side being untouched, Lightness method in the second block, Luminosity method in the third block, and Average method in the block on the right.
Download the zip file, open the zip file by double-clicking on it and copy/paste the script to your GIMP scripts folder (ie...C:\Program Files\GIMP-2.0\share\gimp\2.0\scripts). Refresh your scripts and you're ready to go. The script can be located under the Colors > Desaturate All Layers...
Let me know if you have any trouble or need additional information.