Sorry for the delay on this guys, but I had to track down the manuscript!
The Games Designer's Companion is indeed compiled from a variety of sources - below is a quick (if not 100% complete) list;
Gladiator – Sands of Death, Copyright 2001, Mongoose Publishing
Spycraft, Copyright 2002, Alderac Entertainment Group
Seas of Blood, Copyright 2001, Mongoose Publishing
Dragonstar: Starfarer’s Handbook, Copyright 2001, Fantasy Flight, Inc
Babylon 5 Roleplaying Game, Copyright 2002, Warner Bros., Inc
Experts, Copyright 2002, Skirmisher Publishing.
Heroes of High Favour – Dwarves, Copyright 2002, Badaxe Games L.L.C.
Quintessential Rogue, Copyright 2002, Mongoose Publishing
Heroes of High Favour – Half-Orcs, Copyright 2002, Badaxe Games L.L.C.
The Judge Dredd Roleplaying Game, Copyright 2002, Rebellion A/S
T20 – The Traveller’s Handbook, Copyright 2002, Quiklink Interactive, Inc. Traveller is a trademark of Far Future Enterprises and is used under licence
Nyambe, Copyright 2002, Trident, Inc.
The Quintessential Witch, Copyright 2002, Mongoose Publishing
The Quintessential Dwarf ,Copyright 2002, Mongoose Publishing
The Quintessential Monk, Copyright 2002, Mongoose Publishing
The Quintessential Paladin, Copyright 2003, Mongoose Publishing
Villains, Copyright 2002, Bastion Press, Inc.
Monster, Copyright 2002, Alderac Entertainment Group, Inc.
Dragons, Copyright 2001, Alderac Entertainment Group, Inc.
Tournaments, Fairs, & Taverns, Copyright 2002, Natural 20 Press
Beyond Monks, Copyright 2002, Chainmail Bikini Games, Ltd.
Swashbuckling Adventures, Copyright 2002, Alderac Entertainment Group.
The Quintessential Fighter, Copyright 2002, Mongoose Publishing
Kenneth S. Hood 2001, All Rights Reserved
Black Flags – Piracy in the Caribbean, Copyright 2002, Avalanche Press Ltd.
Rookie’s Guide to Criminal Organisations, Copyright 2002, Rebellion A/S
Slaine, Copyright 2002, Rebellion A/S.
Path of the Sword, Copyright 2002, Fantasy Flight Publishing, Inc.
Wild Spellcraft, Copyright 2002, Natural 20 Press
Traps & Treachery, Copyright 2001, Fantasy Flight Publishing, Inc.
Deadlands d20, Copyright 2001, Fantasy Flight Publishing, Inc.
The Tide of Years, Copyright 2001, Michelle A. Brown Nephew
Seafarer’s Handbook, Copyright 2001, Fantasy Flight Publishing, Inc.
Just a smattering!