Okay...this is what I said under the other Living Greyhawk Gazetter thread:
Buy the Living Greyhawk Gazeeter.
It will have everything you need to know about what happened between CY 576 to CY 591.
DON'T buy the D&D Gazeeter. The 1983 boxed set has more general information than that product does.
However, it also depends on how much you wish to alter things in Greyhawk. If you make Greyhawk "your own" world and barely rely on published material, don't by either. Yet if you wish to stick strictly to Greyhawk "canon" then by all means, buy the Living Greyhawk Gazetter.
The LGG gives a lot of background information to the various regions of the Flanaess. And yet it leaves room for the DM to customize it, just the way I like it...and just the way the 1983 set does.
I hope that helps.
Ulrick