The Green Normalized Difference Vegetation Index (GNDVI) is an index of plant "greenness" or photosynthetic activity, and is one of the most commonly used vegetation indices to determine water and nitrogen uptake into the crop canopy. See the "About NDVI" section for more information behind the science of the measurement but GNDVI basically replaces the red piece of standard NDVI collection with a very specific band of light in the green range to obtain different useful information.