While this book is advertised as system neutral, and this is mostly the case, and while it does not contain an OGL license, this book makes numerous reference that are clearly to the d20 system. These references include DMG, CR (Challenge ratings), hp (hit points), several names spells and magic items and various d20 standard stats and saves.
From the back of the book:
Systems-Neutral Tools for Every Game Master
GM Gems: A Tome of Game Master Inspiration is filled with a wealth of information and ideas to empower every aspect of your game. Never run boring, vanilla games and never be caught flat-footed!
Inspiration and Perspiration
This 64-page collection of tricks and tips is systems-neutral, so you can play it with any role playing game you choose. GM Gems is written by veteran Game Masters and some of todayís best known RPG designers, and includes:
Alchemical Mishaps
Empty Rooms Worth Describing
Extraordinary Campsites
Familiar Creatures with Unfamiliar Faces
Short Encounters for Short Attention Spans
Unique Taverns and Inns
Unusual Holidays
100 Unique Treasures
And much, much more!