How much/what kind of stuff is in the GMG that doesn't appear in the PRD/d20PFSRD?
All the useful tables (which I really wish WERE on the PRD, simply because it's much easier to reference a simple-formatted Website than an overly elaborate, pain-in-the-ass-to-load .pdf designed obviously for print reading--or a book that you keep having to flip through and hold pages open). Hell, I'd rather see only the tables on the PRD instead of the stuff they do have up there, because they're (well and the NPC statblocks) the main part a GM will actually reference during a game.
Plus the first... oh, I guess third or so that's advice for GMS--basics of GMing, typical player types and how to work with them, campaign design basics, that huge page of words that someone cribbed from an SAT vocabulary study sheet. That's what I can think of off the top of my head.