Greyhawk is officially the setting of the new adventure book “Ghosts of Saltmarsh”.

Thanks to a preview thread of Ghosts of Saltmarsh from @NewbieDM on Twitter, we now know beyond all doubt it’s not set in a “generic” setting-less campaign. Nor is it officially in another setting with a sidebar for adapting or placing it in Greyhawk. It’s absolutely, unequivocally, definitively set in Greyhawk.

“PLACING THE ADVENTURE: The town of Saltmarsh is a small, respectable fishing town located in the Greyhawk campaign setting, in the southernmost part of Keoland.”

You can click on the image at right, to see the full image.
The specific pivotal sentence reads,
“PLACING THE ADVENTURE: The town of Saltmarsh is a small, respectable fishing town located in the Greyhawk campaign setting, in the southernmost part of Keoland.”
It then goes on to describe how the adventure can be used in other campaign settings like Eberron, Forgotten Realms, and Mystara – much in the same way Greyhawk has been included in other adventure books in 5e.

Since the announcement of the new book, which has been greatly vaunted for containing new vehicle rules for 5e D&D, there’s been a lot of rumblings of complaint on the ‘net.
Many people have referred to WotC “stealing” or “plundering” settings for plots, places, storylines, etc. It’s likely that this comes from a place of concern from fans that their favorite settings aren’t ever going to be in print again, and they don’t like seeing the setting they like having content previously from that setting being used in other settings.

While this is a completely understandable feeling, it seems to be fairly unfounded, and more a knee-jerk reaction than anything else. While at GaryCon XI, we had the privilege of discussing Greyhawk and other setting’s appearance in the DMsGuild with Mike Mearls, both publicly at a seminar panel and privately in an after-lunch conversation.
Mike was very clear in saying that WotC is absolutely going to continue releasing settings

We’ve tried to be very vocal advocates for this book recently, after it being revealed that Ghosts of Saltmarsh included names of places like “Sea of Gearnat”. It was very, very clear (at least to GHO) that WotC had no intention of taking Saltmarsh entirely out of Greyhawk.

It was discussed by Kate Welch on DragonTalk (at about the 11:10 mark), with her saying that the adventure would be playable in any game or setting, without using words like “generic”. She did say that it “isn’t a pure Greyhawk book” and that it was “extricated from that setting and made it agnostic so that you can put it in whatever setting” while having “lots of little nods”. She clarified this by saying “This isn’t a setting book, it’s an adventure book that happens to have a setting included … a little mini-setting”.

But, it has been made very clear in later discussions from developers writers, and most specifically by the screenshots from @NewbieDM, what she meant wasn’t that it’s not “pure Greyhawk”, it’s just that it’s not a pure setting book. It’s not about the World of Greyhawk setting. It includes a small, isolated view of a portion of Greyhawk, set in Salinmoor in Keoland. It doesn’t cover all of the Flanaess (or any other portion of GH), and it’s not even a complete look at Keoland, Seaton, or Salinmoor.
It’s just enough to run the adventure.
But, the Greyhawk which is in there … is “pure Greyhawk”.

But, all of that having been said, this certainly does make it seem likely that this being the official setting of the book may indicate the an increasing likelihood of a release of the World of Greyhawk on the DMsGuild. Not that it means it’s definite, but, at the very least, it increases the likelihood it’ll come eventually.

[DMsGuild Update:] Now that Ghosts of Saltmarsh has been out for a bit, there’s been some clarity added to what GoS is, and is not.
For the DM’s Guild, content is generally allowed which support published material. Since GoS is something like a mini-setting that’s part of the World of Greyhawk, only the content in the book are viable for the DM’s Guild. I.e. – generally speaking the area covered in the overland map in the adventure.
Greyhawk as a whole, or even just other parts of Keoland aren’t within the scope of the adventure.
Chris Lindsay, who is the D&D Product Manager and “Architect of the DM’s Guild”, confirmed the distinction between the setting and the adventure on Twitter: