From publisher blurb:
This tome essentially combines what you have come to know as both a monster's manual and a game master's guide. It contains all you need as a Game Master to challenge your heroic Myth & Magic characters.
Monsters: A slew of over one hundred canon monsters comprise a chunk of the Game Master's Guide. In contrast to how the monsters are represented in the Starter Guide, the GMG will dedicate at least one full page to each monster, bringing back the same look and level of content seen in 2E. As a Game Master, you will have the crunch and fluff to roleplay your monsters to the fullest effect. Take the ankheg below as an example. By reading its entry, you can start to envision ways in which to plague your adventurers with this monster:
Magic: The book is chock full of magical items. All the greatest magical items from 2E make an appearance (ahem, sword of sharpness anyone?). We have also scoured the many encyclopedias of magical items to pluck out a few items of lore we had so much fun using.
And More: The GMG contains more than monsters and magic. It contains advice to build and play NPCs and villains, a random dungeon feature generator, advice to run dungeons and other locations, sample NPC and villain archtypes, diseases, curses, poisons, and more.