The material is available in several formats on platforms like DriveThruRPG: Gunderholfen: OSR Megadungeon Adventure Review
, the PDF is often cited as one of the best values in the OSR market for the sheer volume of content provided. Modular Design
You can find the official digital versions and bundles on DriveThruRPG : : The core 400+ page book. Gunderholfen PDF Bundle : Includes the Encounter Pack and Map Pack.
Gunderholfen is a city in the (part of the Flanaess in the World of Greyhawk). While it existed in early lore, it was significantly fleshed out by Robert S. Conley in his "Majestic Wilderlands" supplement.
For ease of use, G. Hawkins released separate PDF and softcover booklets: Gunderholfen Encounter Pack