They didn't just plan. Before his death Gary signed contracts with many authors to write 24 manuscripts, each of about 30,000 words, expanding one district of Yggsburgh. Yggsburgh would become a place so fully-mapped and richly-detailed that you could move through it like a dungeon: mapped in ten-foot squares, each room keyed, each inhabitant statted out, working under Gary's guidance. Most of these manuscripts were delivered, and the majority accepted by Gary for publication (mine certainly was). Troll Lord Games actually published about eight of them IIRC, some in print and others only as .pdfs.
Then Gary died and the product line vanished as if it had never been. I've got some of the manuscripts that were shared between authors for various reasons but by no means all. They're generally rich in creative detail and would be hard to use in practice just because of finding the information you need among more than 700,000 words of text...