The Gamemaster's Guide gives information and advice about running a Four Colors campaign.
The first section: "Rules and Stuff" gives some rules for special situations and methods of character advancement.
The second section: "Gamemastering in the Golden Age" gives information on running a Four Colors campaign. Conventions for the genre, to help set the tone of the game. How to set up Issues (session/scenario), Series (campaign), and Master Villains.