The GM's Companion serves as a handy addition to Outbreak: Undead that really does add a new degree of complexity to the risk and reward of missions in the zombie apocalypse. While it's not exactly going to turn your game upside down with a ton of new and unexpected things, it provides a good guide of what you can expect to be doing in the zombie apocalypse and things that can be put into play that won't beat the players' characters to a pulp in seconds.

Basically, the GM's Companion serves as a guide to what you want to do to keep a game smooth and on track, and allows you to use random generation to figure out stuff to put in if you lose track of where things should be going. It's not giant, but it's got enough stuff in there to be worth noting as a valuable aid.