blocks = Table[{Graphics[{Darker@Green, Rectangle[{0, 0}, {1000, 200}]}, PlotRangePadding -> 0, ImagePadding -> 0]}, {i, 6}] myGrid = Grid[blocks, Spacings -> {0, 0}, Frame -> All] 
If you are still seeing whitespace as per your comment then please post a pic and the exact code that you used.
Edit
As per reply to Mr Wizard. At the max (max of widow element) magnification of 300% I get

Using Magnify of 6 (600%) I still do not see a problem:

I'm on OS X 10.9.4 and Mma 9.0.1