I've got the "Gazetteer of the Known Realms" - aka the World of Aereth (both in print and PDF) and it is worth every penny I paid for it. It comes with a ton of maps, a Gazetteer, a GM's Guide, and two very good DCC adventures.
The Gazetteer is 100% game system agnostic.
I don't have the DCC RPG yet, but in looking at the GM Guide for the World of Aereth, there is only about 30% of it that is 3.5-specific (ignoring the listing of the domains for the pantheon, there are 25 pages of monsters, and 20 pages of feats, spells, and magic items in a ~ 135 page GM Guide).
The maps are well-done - nothing too fancy, but very well done. The one down-side (or upside, depending upon your bent) is that they are not hex-mapped. No big deal for me since I just print out a hex grid onto clear plastic and overlay as needed since there are separate maps for the players that are less detailed).
The single most important part of the GM Guide is the instructions for creating 0-level PCs, so even back then they were thinking about the funnel.
Second most important part of the GM Guide is the adventure path suggestion - they provide 5 different suggestions for how to link all of the DCC adventures published up to that point into a coherent adventure path.
Some people don't like the campaign setting because it is too "generic." I consider that to be its strongest point of all - it has enough detail to flesh out somethings, but not enough to crimp a GM's style. This is what the original World of Greyhawk Folio was like.