One of my co-workers recommended the Harvard Nutrition Source as a place to look up all about Nutrition. In general I find it inferior to Mark's insights, but it's interesting to see how the mainstream medical world is turning around to accept diets and fitness more in line with the Primal Blueprint. That said, I was also looking for a better Glycemic Load index than what I had found at the Harvard School of Medicine, sponsoring the Harvard Nutrition Source site, so I followed one of your links and I find the HSM site to be a bit more complete:
Glycemic index and glycemic load for 100+ foods - Harvard Health Publications
Also, you can generally expect 100g to be equal to almost 1/4 cup. This varies because 1/4 cup is defined as the standard measure for 4 ounces of water and that converts to 113g (or 113ml), strictly speaking. That's not much help with watermelon, but it might give you the picture.