If they just concentrate on the Outer Planes, it would be doable, as it was done pretty well in just 62 pages in the 3e Manual of the Planes. 30 pages for Sigil, the Outlands, the Factions, and general setting rules might be a bit tougher, though. But then again, it's specifically a "gazetteer", which would indicate that it will concentrate less on rules and more on locations...
Then again, the game rules parts might be shunted off into the adventure. I'm not terribly worried if the adventure is bare bones, as the Vecna adventure they've mentioned sounds as if it will be doing a lot of plane-hopping anyway...